If you're a bit more flush try one of the Lotus 7 clones (Caterham, Westfield etc), they range from saloon to V8s via performance bike engines so can challenge the 'Big Boys' if funds permit.
You don't have to be flush to get yourself a kit car. Locost do a kit where you can build something up for £500, all you need is written off 2-litre Sierra for donor parts. Caterhams and Westies cost more but there are loads of these things out there, Quantum, Tiger, Dax, Formula 27, Vindicator, Mac#1, there are literally hundreds of models to suit all budgets.
I've got a Robin Hood kit car, you'd be hard pushed to find anything for £4k that will beat it on the road or the track. It was up for sale until we spent a grand on sorting the suspension and decided to keep it for a bit longer.
I'd say go for a kit car, you can do it as cheap or expensive as you like, anything from a £500 Locost to a £300,000 Caterham Levante. Caterham have been banned from LeMans after turning up one year and whiping everything else above them. Kit cars are serious bang-for-buck. They're so simple, easy to work on, there's nothing to go wrong, parts are everywhere, they're so light, handle well, accelerate like nothing else and the thrill of being out in the open air. The only downside is the weather in this country!