Ooh well done for taking my advice and not going to Belfast
Do a bus tour and then you can take in Big Ben, Buckingham Palace etc
I saw Wicked in June with my girls and it was absolutely brilliant, would definitely recommend it and Billy Elliot is next on my list
If you like Harry Potter I would defo do the studio tour - it's in Watford but you can get a train there in twenty minutes from Euston and there is a shuttle bus
Have fun