Obviously London would be a must you could ride the London eye,
Explore the London Eye
You could visit the Tower of London
Historic Royal Palaces > Home > Tower of London
Wales is beautiful a couple of hundred miles from London, Scotland too but a bit further.
There is loads of historical stuff, also places like Madame Tussauds the waxworks.
Paris is quite expensive but worse than that it is full of eastern european beggars, when we visited it made the trip a nightmare, it was almost like Victorian England with women with small children on their backs begging money or virtually forcing you to sit while they draw you for money!
Even if I were mega rich we couldnt have given to all of them.
The building were beautiful though, loved the eiffel tower, didnt have the courage to go to the top!