I've been moving from UK shops to European ones, Luggage, the prices have increased but so has the quality, I've found some amazing linens now but I assume it's not worth it from the UK.
In the UK I bought from jobbers, Iana fabrics is quick and has a varied assortment and a free sample service (on their own site, you also get a discount on there automatically on Etsy you're charged and shipping was more expensive), they have some handwoven Harris tweeds ATM for quite a low price for handwoven HT. I would say I'm glad I started out buying cheaper fabrics there because I sort of learned from my mistakes, things that looked good as a sample weren't as weighty as I had anticipated, etc. Even if I could still easily buy from the UK I think I'd have moved on with somewhat better knowledge (although they're great for silks, if have continued buying those there) but I'm glad I didn't make those miscalculations with more expensive fabrics and they do have some really good gems and non boring colours but there's not much information (weight, composition you often need to ask, etc). There's a beautiful silk/wool sienna fabric on there that I'm kicking myself for not purchasing pre Brexit 🤦
I never ordered but TheFabricMan on Etsy has some good wool suiting deals, mostly boring colours but occasionally a punchy colour no one wants.
Croft Mill always looked good to me too which I think is also a jobber but of branded fabrics.
Amofabrics is quite interesting, you buy from manufacturers directly, they're cancelled orders or deadstock. It's from all over Europe but the best stuff seems to be in the UK and they often didn't send abroad (even pre-Brexit)
there were some nice linens on there IIRC. Just be mindful some things are furniture fabrics, especially with linen that's a difference in feel. You could request samples but they don't always have them for everything.
Black Horse Lane also sells fabric under "Haberdashery", thick cotton twill and denim, they have a scant few non denim colours, they have an interesting light-ish green that could work well for spring.
And Joel and Son have some gorgeous things and a lot of colour. It might be worth getting some samples just to see what top quality looks like when you put samples side by side. But I will warn, having done this, you're suddenly justifying extortionate prices...