I think I have it narrowed down to three options: Castle (Windsor or Leeds, with Windsor more likely, as we'll be relying on public transportation), College (Oxford or Cambridge) or Bath. I've almost managed to write Bath off the list...but everytime I do, I start thinking...but it's historic, it's different.
So...given that we're relying on public transportation, want something that will fill a day without making us feel like we've run a marathon, and will already be seeing (probably, still ironing out plans): Tower of London, British Museum and Library, Kensington Palace, St Paul's and Westminster, as well as a number of Cathedrals/Versailles, when we go to Paris, which would give us the most bang for our buck?
(There's 4 of us...2 adults and a 10 and 12 year old- both girls. They all hold a fairly equal interest to us...with the girls slightly more interested in the castle or college options...but the 12 year old did recently do a unit on Roman history, so Bath would probably be interesting to her as well)