Probably whatever is local, Castle Combe is just fine, with some initial instruction.
However, be wary of airfield circuits. They are not as friendly as a lot of people would suggest:
There is often more loose gravel and the circuit is usually made up from cones.
- If you hit a cone you can damage your car
- The tight complexes can be more difficult to make out
On a proper circuit, the track is clearly defined and the object is to stay on the black stuff. On an airfield circuit, it is usually much tighter and more difficult to 'see' the track.
Having said that, airfield circuits can be fun, but probably not the best place to start.