We are members of English Heritage, and have been pleased with it. Students (that includes college students) get a reduced rate, and kids go free if you have one adult membership (including as a student). So it definitely made sense for me to join, but since most of our sight seeing has been to EH sites, it’s made sense for Heather to join as well. The benefit of being with EH is that you get 1/2 price off for Historic Scotland and Cadw Wales the first year, and they’re free after that. Places EH runs that we’ve really enjoyed are Finchale Priory (just outside of Durham), Lindisfarne/Holy Island, Stonehenge, Belsay Hall (a manor, garden, and castle about 1 hr from Durham), Scarborough Castle, Richmond Castle, to name a few. In Scotland, the castles at Edinburgh, Stirling, and St. Andrews are all run by Historic Scotland.
To get places we’ve found that BritRail Passes are convenient for when visitors are over. They give you flexibility with train times and are reasonably priced, especially for groups. Also, it definitely pays for college students to get a Young Person’s Railcard–it’s £24/year, but you save about 1/3 off rail tickets. (You can still get it if you are 26+ as long as you are a full-time student).