Regarding 18 Greens & SAS' mention of DEFTS at RAF Barkston Heath, civilian FI courses tend to be available if there's spare capacity alongside the military task.
The training is second-to-none. A meaty variable-pitch 260hp Firefly M that'll do brakes-off to 10000' in under 10 mins. Full inverted systems, +6/-3g, with some pretty gyroscopic aerobatics thrown into the G/H sorties if you so wish. Instructors hold mil A2 or A1 cats, are often ex-CFS examiners, and have a huge amount of experience. It's a disciplined and very punchy environment, but the flying is great fun, demanding yet rewarding.
A wealth of knowledge & war stories in the crewroom as well. Not every flying school has two instructors current on WW2 piston fighters!
Captain Tim Wooltorton is the man to speak to at Barkston, he's the Chief Pilot.
Do it, you won't regret it.