Online retailers

Here's a list of useful UK fabric retailers selling some specialist fabrics and notions:

Bag making supplies

U-Handbag (also has a useful tutorials page)

Independent pattern companies

Colette patterns (one of the better known independent pattern companies, also has a wonderful blog)

Christine Haynes

Hot Patterns (specialising in on trend patterns) 

Pattern Runway

Salme (a good variety of contemporary yet feminine patterns)

Sewaholic patterns

Organic cotton

Fabric Rehab's organic printed cottons

Underwear supplies

Corset Making (don't be fooled by the name - they have loads of bra and knicker-making supplies)