Yup, STM has it - Schumacher sell the base car and separate hop-ups and Zen/Mardave sell the base car with the alternative of a 'race' kit that has all the hop-ups in.
Any kit you find on the Schumacher/Zen/Mardave/Ore websites advertised as for GT12 will be within the BRCA rules and so all clubs accept them for racing.
Schumacher, Ore and Mardave sell kits in pieces that you build, the Zen comes built. Take your pick. If you are not sure how to build and set up these cars, especially a differential, the Zen has a lot of appeal. If you like tinkering and sorting a car and have some experience or a club full of helpers, then the others might appeal more.
You'll need a 1S-compatible speedo, a 13.5T BL motor and a servo to fit your car - see notes above from a lot of people on that! Pick a reasonable 1S pack, a lightweight body shell (Kamtec and Protoform ones come with a very good, free sticker sheet - others don't!

) and some tyres and off you go!
If you're stuck for choices or recommendations, come back here and we'll help.