Got 18 out of 20 now.
Still stuck on 15 and 16.
Got the last few by thinking 'That looks like a certain area of the peak', 'That looks a bit Scotlandish', 'That's on the coast. What well known caves do I know that are by the sea?'
15 looks a bit Mendipy or Forest of Deanish, to me. 16 not got a clue.
I haven't got anywhere in N. Ireland so maybe they are there?