I have a few that I use. My absolute favourite vendor thus far is
Chefs Vapour - I've yet to try a
juice of theirs that I haven't liked, and keep two of their flavours in bulk to ensure that I don't run out. You have the option of altering their PG/VG blends on their site, although it's not full control.
I also really like
The Alchemist's Cupboard - not cheap, but the liquid is fantastic! Some of his flavours are actually a little too real for my tastes - his Turkish Delight is spot on, but not something I can
vape for long.
Royal Vapour's Mystic Juice range has some real winners in it - their Vanilla Custard is fantastic, and I also like their Mirage a lot.
Triphammer is also very good, although his juices are quite sweet and... heavy. It depends on what you like, though. His Fizzy Trifle is brilliant, based on the famous Grant's Vanilla Custard.
Joose Juice are very reasonably priced, and the service is great, although I've found his juices to be a bit hit and miss. I love his Toffee, and his Soft Mint, and I like Key Lime and Strawberries and Cream from him, but his Apple, Blackberry and Raspberry flavours didn't do much for me; they were all too artificial for my liking.
I hope my little list of vendors helps - I have some other UK-based vendors on my 'to try' list, but those the ones I've used and been very impressed with.