As NigelOnDraft mentioned, the Jet Provost is a good starting point. Similar to this is an Aero L-29 Delfin jet trainer, costing around £50-80,000 but I expect the running costs on this (or any other fast jet) would be truly horrendous. Next step up would be an Aero L-39 Albatross advanced trainer/light ground attack aircraft costing around £300,000 for a decent model. There are also the classic Vampire and Venom fighters, and the Folland Gnat of which I believe there is one for sale at the moment. Finally, I believe that one of the largest fighters you can fly in the UK is a Hunter. Talk to Delta Jets at Kemble, they operate Hunters, Gnats (I think) and Jet Provosts.
Ox