I would do if there were more national trust propeties around here. But there are not and it's not worth it.
However, I am a Historic Scotland member and that is absolutely brilliant. It's £36 a year for a single membership (no idea on the family, couples, etc) and it's so worth it. Historic Scotland has all kinds of events, which is great as a member because its free, but you'd normally pay £8, £10, £15 to go to these things.
We've been to a few midevil things and then over the weekend to a Celebration of the Centuries -complete with a Spitfire display. I've had various folks visit and its great to just show the card to the properties. You also get 10% off in the gift shops, cafes, etc.Not to mention you get into English Heritage properties for free (first year member is only 1/2 off though)
We will often just pop into Urqhart Castle to use their cafe, because its lovely to have soup (at 10% off!) whilst looking over castle ruins and Loch Ness or into Ft. George for dolphin/whale watching.
I don't know if English Heritage offers the same kind of things, but its definitely worth it if they do!