I think the most challenging airfield in the UK for me was Out Skerries in Shetland; it is short and not flat. In fact, if you get yourself up there you will find that there is a whole bunch of interesting island airfields all within 30 mins flying time of Tingwall (EGET) which can be used as a base. Also Fetlar, Foula, Whalsay, Papa Stour and Fair Isle are all interesting to visit. Superb scenery and deserted airspace (mostly); a real treat. The weather can be a challenge too ! And en route to Shetland, why not call in at some of the Orkney airfields too ? Many of them see very little GA.
SI.