I have been a member of English Heritage for a number of years and it has been one of the best thing I could have done.
Once you purchase membership, the abundance of free days out is endless.
If you purchase your membership through Tesco it really is free!
I have not been disappointed with anywhere I have visited and if you are into stay-cations you will never be too far from somewhere to visit.
If you need to speak to anyone regarding your membership there is always someone willing to help with any issues, even if you have forgotten your card and are already on your way to a historic sight.
If you have children they can be added to your membership for free and English Heritage run a number of things at their sights to keep children engaged.
Again, one of the best things I could have done, is join English Heritage.