Well, I am biased (but then so is every one else on here!) and I would definately recommend the Monoposto Racing Club as a good place to start in single seaters. Very cost effective, a good bunch of people to race with and good driving standards/ respect for other drivers.
Cars can be bought cheaply (I know of a Vauxhall Junior for sale for about £3750 in good condition), are fast and yet running costs are low.
Check out
www.monoposto.co.uk or PM me if you want more details including a real break down of the costs of competing.