Yeah there's a lot of great sellers on etsy for all manner of ABDL items, and ebay. I'm not gonna link a whole bunch of stores (on my phone right now) but there is a fair bit out there. Probably going to need to look international though as there are not so many based in UK, though etsy has the biggest choice there.
I'd only use Amazon for disposable nappies though or toys and actual baby stuff though, the abdl clothing and things on the UK Amazon site are not the best and not a great selection.
There are many other great sites out there depending on what you are looking for and how much you want to spend, for instance privatina has some great clothing with high quality and custom made sizing but is expensive.
It can also be useful to look at SEN or mobility/disability care shops too for adaptive clothing (onesie etc.) Or other things as they often have clothing and things like dummies, bottles or sippies, bibs etc. In larger sizes, limited selections though.