Oh yes, AmoThreads! @excanuk and also Fabrics at Play. They keep tempting me with emails of luscious fabrics. But not cheap, especially with postage & duty added in (damn Brexit).
I'd second Goldhawk Road.
I also like Dalston Mill Fabrics - it's off Dalston Market sdo it's fun wandering through the market - just opposite Dalston train station on the London Overground line.
Only thing about the shop is there's so so much there that it's a bit hard to browse. And some staff (the men) are a bit grumpy & transactional. But I have several lengths of 100% linen from there, and the dresses I've made from them always get compliments - the pure linen is around £13 -14 per metre (a bargain!) and gets lovely & soft as you wash it, but irons up as only linen does. I had a good browse around the room of wools at the back - great stuff there too.
Also there are a couple of fabric shops clinging on in Berwick Street in Soho. Sadly, they are disappearing from there. The Cloth House is a good place to browse, and their prices are over a reasonable range. Borovicks is fabulous - I got 100% wool crepe from there years ago for a song - it was deadstock from Jean Muir's factory - it's beautiful! There are a couple of other shops as well, but they are all a bit showy & pricey - silks and bridal fabrics I would just never buy (and I buy almost everything!)
And if you're ever in Paris ... there is a street of fabric shops (almost warehouses) in Montmartre, in sight of Sacre Coeur - beautiful beautiful silks and wools.