Large range of fabrics, mainly for dressmaking. 50-52 Goldhawk Road, London W12 8DH, Shepherds Bush area.
Lots of home furnishing fabrics, very large range. 51-61 Wigmore Street, Marylebone, London, W1U 1PU.
All the Fun of the Fair - this shop closed
A-Z Limited Fabrics
Nice range of fabrics, many quite affordable. Quite cramped. Shepherds Bush area.
Beadworks - Tower Street, Covent Garden
A very large range of beads. A good range of jewellery making supplies.
The Berwick Street Cloth Shop - and Broadwick Silks
Two shops on Berwick Street, Soho. Cloth and silk fabrics, plus buttons and ribbons etc. At No 98 and No 47.
This shop moved to: B J House, 10-14 Hollybush Gardens, London E2 9QP (it was at 67 Columbia Road, E2 7RG). Quilting fabrics, haberdashery, books. Beautiful crafty items carefully chosen.
High quality fabrics. Berwick Street, Soho
Buffys Beads - this shop closed
The Button Queen
Very eclectic range of buttons. 76 Marylebone Lane, Marylebone area.
Good range of fabrics, quite cramped and busy. Shepherds Bush area.
The Cloth Shop
Unit 12, The Market Hall, Wood Green Shopping Centre, High Road, London N22 6YE
Between the Primark and New Look units, in the Market Hall area.
Massive range of beads. I believe they used to be in Beak Street, at the bottom of Carnaby Street. Now Marshall Street, West End.
Dalston Mill Fabrics
69-73 Ridley Rd, London E8 2NP (the post code on their website is wrong, it is not 3NP)
An Aladin's cave of fabrics! Has got to be seen to be believed. An incredible amount of choice. Plus haberdashery and all kinds of materials. A must go.
52- 54 Lavender Hill, Battersea, London SW11 5RH. I love their thoroughness in putting a label on every bolt of fabric showing price and fabric composition. Other shops could learn from them. Dressmaking and soft furnishing fabrics.
Fibres and yarns, mainly for weaving, also for knitting, sold by weight. Spindles. Lots of books too! On Seven Sisters Road, near Finsbury Park tube station.
Yarns, needles and magazines. Based in Lower Marsh, Waterloo.
John Lewis, Oxford Street
Various yarn brands, some knitting/crochet accessories. Sewing machines. They also do quilting fabrics and other crafts supplies. Fabrics. Forth floor next to the toy department.
Klein's London Showroom
Trimmings, fabric dyes etc. Very odd opening hours. Noel Street, Soho.
Just been (Jun 2010): they've got loads of ribbons/braids/trims. Any kind you want: patterned, rouched, elastic, rubber, leather... you name it, they've got it.
PS: There is another trimmings shop opposite Klein's. I am sorry I don't remember the name.
Knit With Attitude
228b Stoke Newington High Street (entrance on Northwold Road) N16 7HU. Not been yet.
The London Bead Company
Beads and needlecraft supplies. Based in Kentish Town. Very good product range, could do better at customer service. Maybe I caught them on a bad day.
The London Bead Shop - Earlham Street, Covent Garden - this shop closed
Gorgeous yarns. Based in Islington. As of June 2010 at 15 Camden Passage, Islington, nearest tube: Angel. Rather expensive.
MacCulloch & Wallis
A variety of fabrics. Lots of sewing supplies upstairs! Also sewing machines. Dering Street between Oxford Circus and Bond St tube stations. More pricey than many other places.
Good range of yarns (prices, colours and weights), lots of other goodies too! Well worth the short bus trip on the W3 or W5!
Peter Jones, Sloane Square
As John Lewis but a more limited range of yarns and fabrics.
Quilting Bee - This shop closed
Rolls and Rems Holloway
There are another two branches in Lewisham and Edmonton that I haven't been to yet.
21 Seven Sisters Road, Holloway, London N7 6AN. They sort their fabrics by colour, love that. Lots of choice.
170-172 Old Brompton Road, London SW5 0BA
A massive maze of selling space downstairs (must be seen to believe). Lots of Liberty style fabrics at about £17 a metre. Some nice viscose.
Two shops in Brixton. One at 48 Atlantic Rd, London SW9 8JN, the other opposite.
Very good range of dressmaking fabrics, and also other materials and haberdashery in the second shop.
Silks. Pretty fancy. Gorgeous fabrics but expensive. D'Arblay Street, Soho.
Tricolette Yarns - this shop closed 
More affordable fabrics. Quite cramped. Shepherds Bush area.
Woolcrest Textiles (on FB)
6 Well Street, London E9 7PX, 020 8985 3686. A warehouse full of fabrics. They are said to open Saturdays now too [July 2014], call before you go just in case.
Wools and Crafts
Yarns and lots & lots & lots of crafts supplies. Blackstock Road, Finsbury Park. Nearest tube: Finsbury Park or Arsenal. Rather odd opening hours. Do not do any exchange once purchased.
I will add more shops as and when I get to know them. I have been to most of these.