Hey there, sounds like you've got some good suggestions so far. Can't help but wonder if you've ever seen a London show? There are some fantastic shows at the mo, I recently sawe Footloose and thoroughly enjoyed it! Also, we will rock you, chicago and wicked are great. Sound of music will be running soon. I booked my last theatre break with www.show-and-stay.co.uk and it was really good value (think it was 49 each including the hotel). Might be worth checking out?
Also, if you get a chance visit the food hall in harrods - their cakes are amazing, a typical British pub (the hereford arms near gloucester road tube station is very nice) and downstairs in covent garden at lunchtime by the coffee shop there is often great live music - opera etc. Have a fabulous trip!