There is a Currency Exchange next to the Tower Shop.
Walk on down to the River Thames and you will see Tower Bridge. The Guide explained that the high-level walkway was intended to be a pedestrian crossing when the bridge was raised for boats to go through, but the idea wasn't really popular. (Probably too many steps!) If you visit the Tower Bridge Exhibition there are often events happening on the high-level walkway, and you can get some amazing views of London from up there.
I thought it would have been useful if the Guide had pointed out some of the buildings you can see on The South Bank of the river, like City Hall as it is such a striking building.