Of the airfields I know (in the UK as a whole).....
Sleap is nice, but pretty dead except at weekends, sometimes anyway. It's also not the greatest plance for sightseeing, aerial or otherwise.
Welshpool ditto.
Wellesbourne...I'm inclined to agree with BEagle on this. Small enough to be friendly, but several flying schools. Usually plenty of people around. Good cafe. Loads to see, as BEagle has said.
Shobdon - also good cafe and friendly, but a bit in the middle of nowhere for other types of sightseeing
Tatenhill - small, friendly, but cafe only open at weekends and a bit dead in the week. Near Alton Towers and all the scenery and stately homes of the Peak District, so good for sightseeing, especially from the air.
Places I don't know so well, but might be good...
I'm supprised no-one has mentioned Kemble - friendly, great cafe, nice area with Cotswolds etc nearly. Don't know what flying schools are like.
Old Sarum always seems like a real GA airfield, friendly etc, and near Salisbury, but I don't know it well.
I'll post any more if I think of them - I've been to enough airfields so I ought to be able to think of somewhere else.