Everything from tools to buildings and plants supplied direct or through partners. UK delivery free over £15 otherwise from £3.33 for home & garden, depending on number of items.
All kinds of plants at excellent prices and an easy to use plant finder. Also garden furniture, machinery and other equipment. Delivery to most of mainland UK £5.95 (seeds £1)
1500 flower, herb, fruit and vegetable seeds, plants and bulbs. Garden tools & equipment, greenhouses, wild bird and pond care. Delivery £3.95 or 95p for seeds only. 
Garden furniture, awnings, gazebos, BBQs, garden tools, entertaining and ornaments. their web site. Standard UK mainland delivery is £4.95.
Manufacturer of water butts and home compost bins, selling over 200 garden and indoor products. Free delivery over £75, otherwise usually £5. 
All you need for composting, growing and pest control. Equipment, tools, buildings, water butts, pond and plant care. UK mainland delivery £3.95.
Wild bird food, feeders, nestboxes and accessories. Same day despatch on food if ordered by 11am. UK delivery £2.99 per order.
Fair variety but small selections in each group. Seeds, buildings, tools, plants, machinery. Delivery to most of the UK from £1.29 or £5.99 to 30Kg.
All sorts of garden buildings including playhouses & metal sheds. Also spas, hot tubs, furniture, decking and garden play. Free UK mainland delivery on most things
Garden tools and machinery. Also a few general tools and power washers. Free delivery to most of mainland UK.
Flower, vegetable, fruit and shrub plants and accessories plus vegetable seeds. UK delivery £2.95 per order.
Massive variety of plants, shrubs and trees including new varieties. Also links to video advice films. Free delivery over £75, otherwise £5.95
Furniture, buildings tools, BBQ and other equipment for the garden plus outdoor clothing and bird feeding. Free UK mainland delivery over £45, otherwise £4.95.
Garden machinery; outdoor leisure including hot tubs; plants, shrubs and trees; garden furniture, outdoor clocks and water butts. Delivery £3.95
Flower, fruit and vegetable plants, bulbs, herbs etc. grown and delivered by post direct from the nursery in Jersey. Also feed and watering kits. Free delivery.
Garden machinery: mowers, trimmers, cutters, blowers and vacuums. Prices include free next day delivery.
Mowers, trimmers, hedge cutters, leaf blowers and the like from all the leading brands, plus accessories. Prices include delivery.
Everything you need to create, keep and maintain a garden pond including liners, pumps, filters and vacuums. UK mainland delivery £4.95.
Conservatory, cafe and outdoor furniture and games, lighting & heaters, gardening BBQ and wild bird products. Free UK mainland delivery over £75, otherwise usually £4.95
Favourites for the garden and kitchen, including seeds, bulbs, plants, vases and floristry, books and kitchenware. UK delivery £4.95 (seeds £2.50)
Not just sheds, but all sorts of garden buildings, including playhouses, summer houses, outdoor play and furniture. Free delivery to most of mainland UK.
Flower & vegetable seeds, plants, bulbs, fruit bushes and trees, equipment, tools and buildings. Delivery to UK mainland £3.95, (seeds 95p)
Greenhouses and everything for them, also outdoor plant protection, games, clocks and traditionally made garden tools. Delivery £4.99 to most of UK
Flower & vegetable seeds, flower bulbs & plants, fruit plants and trees. Web site has detailed growing guides. Post for seeds £1.25. Everything else £3.75. Plants, seeds, outbuildings and the obvious things, but also the less obvious such as wormeries, composters and water butts. It's all here!
Crocus probably have the best variety of plants available on the internet. They say they have over 4,000 varieties. Their web site is very easy to use, so to find the one out of 4,000 that you want is not difficult. Although their definite strength is in plants, including vegetables and other kinds as well as flowers, they also sell most other things you could need for the garden, from buildings to Barbecues and outdoor chess. Crocus do not deliver to the islands around the UK, but will deliver to most of the mainland for a fixed charge of £5.95, or £1 if your order is exclusively seeds.
Gardening Direct sell plants, shrubs, bulbs and seeds. They have a very handy plant finder to help you choose and a growing guide to make sure you get the best from your purchase. Delivery is £2.95 per order of any size. Gardening Express also have plants, shrubs and just about anything else that grows with some good offers on a small number of varieties and delivery is free if your order is over £75. Unwins specialise in flower and vegetable seeds, flower bulbs & plants, fruit plants and trees. They also have useful growing guides on their web site and frequent bulk-buy discounts. Delivery costs £3.75 per order, or £1.25 if your order is exclusively for seeds.
Even Greener manufacture composters and water butts. They have a massive selection in all colours, shapes and sizes including tanks. They also have rainwater diverters and other accessories to make sure that your garden benefits from whatever rain you are able to collect and store. Delivery is generally around £5 for the order but free on orders over £75. Other specialists are Pondkeeper, who have everything to help you to make and maintain a pond, including guides on how to do it.
Garden Bird Supplies have an enormous variety of wild bird foods, feeders, nestboxes and accessories. Food is despatched on the day same day if it is ordered by 11am. Delivery to the UK is £2.99 per order, with no supplement for the English and Scottish islands. Greenfingers also have bird food and feeding equipment but sell just about everything else for the garden as well. They have sheds, greenhouses, power and hand tools, furniture and a relatively small selection of fruit trees and plants. Even garden ornaments! Delivery is £2.95 per order of any size. The range is nearly as wide from Robert Dyas but not quite as deep and without the plants. Their delivery is also higher at £4.95.
Garden
Machinery
It is worth checking suppliers on the DIY page as well, such as Screwfix, Tooled-Up and Northern Tool, who also have garden tools and machinery at good prices
For garden machinery, Mow Direct, Garden Cut and Mower Warehouse have tremendous range of discounted ride-on and push mowers of all sizes and fuels as well as hedge and grass trimmers, chainsaws, shredders, vacuums, blowers and accessories. Most machines are held in stock and despatched the day after the order is received. The prices shown on the web sites include delivery. These suppliers are very similar but we think Mow Direct just edges it on the web site's ease of use and faster delivery. Prices of many items are similar too, but it is worth checking them all to make sure you get the best deal. Try Gone Gardening, too, for garden machinery but they also do most other things for the garden including hot tubs and plants. Amazon may be better known for books and music, but they sell all sorts and have some excellent prices on garden machinery. They also list other garden products on their web site, some of which they sell themselves, some via links to other merchants. They also have free delivery on everything over £15 that they they sell direct.
For garden buildings, as well as Greenfingers, try Shedstore and Garden Chic. They both have a whole range of buildings including sheds, summer houses and playhouses as well as other things for the garden such as garden games and furniture. They both deliver free to most of mainland UK. Two Wests and Elliott also have a few sheds but really specialize in greenhouses and just about everything that goes inside. They also have things to protect plants outside the greenhouse such as cloches and fruit cages, traditionally made garden tools, garden games and clocks. UK delivery is usually £4.99 but it may be higher to hard to reach places.