(UK) Garden shops on the web:

Architectural Plants

The Balcony Gardener

Buckingham Nurseries

Crocus - one of the best garden shops on the web!

David Austin Roses

Dobies

Dobbies

The English Cottage Garden Nursery

Ferndale Lodge

Garden Boutique

Garden Buildings Direct

Garden Centre

Garden Supplies

Garden Warehouse

The Gardeners Shop

Gardens4YouUK

Gardening Express

Get Digging

The Green Chronicle Shop

Greenfingers

GreenGardener

Harrod Horticultural

Hayloft Plants

Jackson's Nurseries

Jersey Plants Direct

Just Green

Knoll Gardens

Native Tree Shop

The Natural Gardener

Nicky's Seeds

Ornamental Tree Nurseries

Plantabox - neat gardening gifts

Plantconnection

Plantstuff

Quality Garden Tools

Rocket Gardens - the UK's only supplier of Instant Kitchen Gardens  (super cool)

Swings & Pretty Things

Terrapot terracotta pots & planters

Thompson & Morgan

Trees by Post

Unwins

The Urban Garden

Urban Jungle- exotic plants

Van Meuwen

Wiggly Wigglers - a great site for 'green' gardeners!

Wildforms Shop

Gardeners' Box - a directory

The Gardening Website - portal to gardening sites

The UK Directory for Garden Shopping Online