oooooo, loads of places. What's your confidence in assessing fabrics for their content and what you'd use them for?
I'm always looking for bargains/reasonably priced more "high end" materiakl: silk, wool, linene, nice 100% cotton. There's not much point in me taking ages to tailor something really made to measure with crappy fabric.
I also have picked up a fair bit about fabric over the years, just by talking to experts.
When I'm in London, I like to have a mooch in Berwick Street, but only Borovicks and The Cloth House (if it's still there). There are a couple of other shops but they're outrageously priced. Borovicks know their fabrics and will sometimes have treasures - I bought 3 lengths of wool crepe from the shut down Jean Muir factory about a decade ago for not more than about £8 pm.
The other place that I like - except the service is not really sparkling! - is Dalston Mills fabric on the Dalston market. It's a pretty shabby looking shop front but a treasure trove - my current favourite from there is 100% linen decent weight at between £11 and £13 pm. I have some gorgeous vibrant deep aqua and some lovely white for next summer's trousers. Their wool section right at the back is also luscious, but you do need to have a pretty good idea what you're looking for.
Then there's Goldhawk Road - lots of shops, but you really need to know what you're looking for. They tend to put barrels of cheap poly cottons out the front to get people in. Polycotton is OK for making a toile but it's not worth my time to sew ... But at a couple of pounds a metre it's good for a trial of a new pattern, for example. I tend to get that sort of fabric when I'm in Birmingham - I've got some real bargains there, of good quality fabrics, as well as cheapo fabric to make wearable toiles.