I'm not well-versed on all of the online stores specifically targeting the UK but I can tell you that a few months ago I looked for these things to send to England.
I also want to note that there aren't many sippy cups that are designed specifically for adults. It may end up being that you'll want to find a sippy cup at a local store that is a brand (like NUK) designed for actual children. I know some of those sippy cups go up to 10oz or perhaps even 12oz--which is definitely an acceptable size. So, don't let the lack of the word "adult" on the packaging scare you off and make you think you can't use it.
Now, I found that Etsy seemed to have some really, really nice adult pacifiers and sippy cups in larger, adult sizes. Generally, those sellers were from the U.S. but would ship overseas for a higher shipping cost.
There ARE sellers on Etsy that cater to AB gear too. What you can do to see them is go to the main Etsy site and type in whatever search term you want--like the letters AB--and search. On the left side panel you're able to toggle the location and select something like United Kingdom to see only sellers/shops that are located there. It ended up showing me this link of shops for that keyword:
https://www.etsy.com/search?q=AB&loca ... ry=2635167
So, if you have the budget to spend a little extra on them then I'd look that way.
Etsy also is a wonderful place for things like collars if you feel like you want one for yourself. I bought a collar there a few months ago that seemed like it would be comfortable for long-term wear. The seller was in the U.S. but they happily shipped to the UK for a higher shipping rate.
I did another super quick search online (AB UK search in Google) and found things like ABDLmarketplace.co.uk you might want to check out and compare prices through. Do a quick Google search of AB UK and see what it throws your way!