Designers are often quite guarded about their suppliers.
My partner does some fashion design and had a wesbite showcasing them. He had an email from the USA asking where the tweeds were from and he naively told her.
A month later an American had bought the entire lots of fabric he was using. They had dozens but they specifically bought the exact ones. So he had to use others.
So now he never tells anyone.
It is the same with big fashion houses I had to sign what was virtually a 'secrets act' as a fit model! They are very guarded about fabric sources.
Having said that I can suggest Brick Lane in London though it isn't anywhere near as good as it was. Also The Draper's Record is quite a good start:
My partner is seeking a seamstress in Wales atm with a leather machine if any drop in this thread with one then he has some work for you please give me a pm