The nearest airfields to Tun Wells are Biggin, Rochester and Headcorn. The latter are grass so if you insist on a hard runway it has to be Biggin.
The problem with that is that it's more expensive that the other two and much time can be lost at the hold waiting your turn to take off. Try Headcorn, landing on grass really isn't much different to tarmac, and there is always something of interest going on. Rochester takes a little less time to get to and has a choice of schools.
p.s. To get to Headcorn from the Wells, its quickest via Goudhurst and Biddenden (A262).