Every year for four days in September there are Heritage Open Days, which offer free access to properties in England that normally charge admission or are closed to the general public. I found out about this quite by chance last year when visiting Tynemouth Priory. This is a lovely Priory in Tynemouth, which surprisingly is on the mouth of the River Tyne. We got free entry and in addition they had a number of displays with people in costume telling us about the history of the Priory. It was really interesting.
This year the Heritage Open Days are from the 9th – 12th September. You can find out whats on in your area at this link
I really would recommend having a look