get yourself £50k of sponsorship cash and you can get a place with one of the lower half teams. Get yourself £100k sponsorship and you can get a place on the top teams. Minor details will include getting a race license, being able to drive the car competitively without damaging it, and hopefully avoid damaging yourself in the process.
Good luck
ps. theres an other route for one lucky - and very talented - driver to be selected by renault every year to compete in the cup series. If you feel youre up to it, you can put your name down. Itll cost you a few hundred £ to get to the point where they politely tell you youre not racer-material, or if you are, you can go all the way at relatively low cost.