Oscar Schmidt is a well known brand that manufactures good quality budget ukulele and includes left handed ukuleles in their line-up. I don’t know what their availability is in the UK though, but you can find them in several online (European) shops. Kala, Makala is also a well established brand.
If you’re looking for a good ukulele store in the UK I can suggest the Southern Ukulele Store. I’ve never been there personally, but I’ve heard good things about it.