Personally I'd recommend F Vee or FF1600. Both have plenty of cars around for little money and both have large grids. FFirst is a bit of a dead duck. No real club series unless you want to do hillclimbs or sprints (or maybe monoposto).
I have raced F Vee in Ireland and the UK. The drivers consist of experienced hard to beat veterans, up and coming youngsters (this year there were two that came back down from UK F Renault), thoses looking for a bit of a race thrill and every other sort that you will meet. Secondhand cars can be had for as little as GBP£3k. A set of tyres can last 4-5 races (depending on how hard you are on tyres). Repairs are cheap. The entry fees range from around GBP£110 to GBP£170 (Donington). Help is always available (F Vee are the friendliest in the paddock) and people will loan you equipment to get out for race qualifying (if you are unfortunate enough to do damage in qualifying/testing). All in all it is a great class to start in. Most rounds have a two qualifying sessions, a qualifying race and a final. So potentially you can get two races for one race entry fee (or four races at a double header).
You can check out UK F Vee at the following sites:
www.formulavee.co.uk
www.leastone-racing.com
www.vwracing.net
www.750mc.co.uk
FF1600 is thankfully having a bit of a resurgence. Cars are built by the likes of Van Diemen, Swift, Ray, Mondiale, etc... Tyres are not slicks and last for a few races (someone else can give you the exact details). Looks like you could pick up a FF1600 from about GBP£6k.
The one thing that I can't stress enough about buying a car: make sure that you have someone elses advice that knows the history of the car. Too many people end up getting stuck with a championship winning car (according to the seller) that is just a minger. Happens in all classes unless you know the class. Let me know if you need more info...