Yeah, you can easily get to any of those places for a day trip. Stonehenge is indeed disappointing (it's just a bunch of rocks) but you can always go there on your way to Bath or a castle or something.
British Museum is HUGE, so you might want to either do a little bit at a time over a few trips (it's pretty central - I often pop in if I've got an hour to kill in central london, and you can even do this during your "business phase" after work is done), or you could easily spend an entire day just in one section of the museum. It's amazing how much stuff we've stolen from other countries...
Personally, I don't think any of the Palaces or "royal" bits are worth spending too much time in. See them from the outside, maybe wander around in the free bit if there is one, but generally they cater to old nationalists who just love the monarchy, and aren't particularly interesting to someone like me.
I've never been to HMS Victory (that's in Portsmouth, right?) but I would go to Cutty Sark anyway, as it's in Maritime Greenwich, which is one of my favourite parts of London. Not only is it a beautiful place with lovely buildings and a nice little hill with good views over the city, it also has the national observatory and maritime museums, which if you play Civ are probably things you want to see!