In no particular order, if you're in the South East think about the following:
Cromer/Northreppes (north Nofolk) - short runway, can be tricky to spot;
Bembridge, IOW (windshear on approach, and frequently crosswinds);
Clacton - short runway aligned North to South, so frequent crosswinds. Also a public footpath crosses the runway meaning you may have to go-around for dog walkers
. I've scared myself here - be careful.
Popham - dog-leg approach on one of the runways, and rough/narrow runways.
Headcorn - not particularly difficult but a lovely place to be. Lots going on there - including parachutes and vintage aircraft.
Stapleford. Busy training field with a very congested circuit and a half tarmac/half grass runway which is on the bumpy side. Worth a go.
Stowe Marie in Essex - very much an 'aerodrome' in the traditional sense. Basically just a mown field and a WW1 museum. You will need PPR here.
And as a curve ball - Lydd in Kent. Not 'difficult' but challenging to spot, full ATC (but reasonable landing fee) and the danger areas for dungeness power station close by which makes for a challenging approach.