Many thanks for suggestions, everyone!
In the end, I opted for this one:
https://www.chrisbowers.co.uk/product/plum-marjoriesseedling/
I looked at various suppliers of fruit trees and the range of tree varieties is mind-boggling!!!
I was very interested in buying a traditional (often rare) variety, but most of them are not reliably self-fertile and I don't think I have room for more than one tree. So, I've gone for a fairly safe bet which should provide plums for both eating and cooking. I also wanted a late-fruiting tree as that's the time of year that I'll have more time to make jam :)