I did visit several castles in the UK but it was in the film era, so most have people posing in front of the Castle - we only took a few pictures of each castle in them days..... Now, you can shoot hundreds.
I had a National trust card so I could get into most of the English ones for free.
I was seriously annoyed when I realised that my "National" card did not cover Scotland and Wales....
Stirling castle, Edinburgh Castle, are well kept in Scotland.
Wales seems to have a Castle at each turn of the road.
St Michaels mount is also nice/romantic especially if you go by boat.