Looks like a nice place there at Slinfold, Ive been in and out of there once runway was quite smooth, hangars good, not sure about waterlogging, but I dont think there are problems, a friend used to operate there and didnt mention it.
No problem going directly to/from abroad from any private strip IO540, you just have to file your flightplan on the phone or by fax and tell the authorities what you are doing, even ok for going to places requiring special branch. Might be more of a problem if you or pax are not a EU citizen though