Unfortunately most costumes from "costume shops" are very cheaply made.
I know. That is why I wanted to find one with a nice pattern. Then I was going to customise the hell out of it to get the look I wanted. Here's a photo of a before and after of a costume I made a couple years ago.
Have you tried Etsy? You may need to sew your own (or have somebody you know do it).
Surprisingly, the few people I've found on Etsy do not post to the UK
If those 3 options don't work you may have to commission one.
There is one costume company I know that does amazing work. And their prices are quite reasonable.......if you live in America. Once you factor in international shipping costs and customs tax, it makes it too expensive. Grrrr!
I did find a cheap one that I liked the pattern of. However, it's one size fits all and will fit someone with a 38-40" inch. But I have a 42" chest. So the search continues.