Many would suggest Karting however I have a different point of view:
Karing provides intense hard racing, which certainly is a great thrill, however sometime its so hard it is painful if you take it on as a hobbilist. If you are a outright racer in your heart then it wouldn't be a problem.
But I think there're a few plus side with formula Vee/budget racing like westfields or so.
1.A car with suspension and gears is simply different from a kart, it may be personal preference which is better but a lot of my mates, me included who had been karting for some time, do feel that a car provides you more freedom in how you drive it, and because it's less of a assault in your senses you can analyse how you drive and improve better.
2. You get to drive on proper racetracks, get some real speed and most importantly the sense that you are really going somewhere. With the array of tracks in the UK its one big plus.
3. As it's less intense then karting, there's a more laid back and friendly atmosphere around the paddock (i.e. the lawn behind the pits)
4. if you are doing club racing, you might even convert your track ride for street use. When a kart is done with, it's done with.