Croft Mill are superb.
I also Abakhan, but you need to know your fabrics for that.
I've just had some lovely linen from Simply Fabrics (beware, you need to order by the half metre, so if you need two metres for a dress, you need to order 4 half metres.
I don't rate Tilly & the Buttons, or the myriad of other "crafty" styles of dress making. The patterns are pretty basic & won't teach you anything about proper tailoring & traditional, high quality sewing skills - the sorts of skills that Therese and Clare on the Current Sewing Bee have.
Get yourself to Vogue patterns: you will learn how to sew by following their instructions. They have the best instructions of any pattern I've used (have made most of my clothes since I was about 16). You can get good half-price deals from places online sch as Sew Direct.
A lot of fabric shops are set up to get onto the home sewing band wagon, but they're bad. They have limited choices, and do things like charge £11 a metre for poly-cotton (there is a boho crafts shop on my high street that does/did this. Outrageous.
When we can shop in person again, you need to go to places that look like bazaars, and rummage. Teach yourself about fabrics.
I tend only to buy 100% cotton, linen, wool, or silk - it is not worth your time to sew with anything but the best quality. I have a winter coat that is 25 years old, and still going strong.