The Tower of London is fabulous during the day, so I do recommend that you go. However, even better is that every night the Tower is locked with great pomp and it is possible to be one of the few people who witness this. The numbers are limited to around 40 people. It is free to attend but usually booked about a year ahead.
It is said to be the oldest military ceremony in the world and a magical experience. It sounds like a long way in advance, but book it and the time will soon pass and then you will have a London experience that few people see.
We had dinner before in a nearby restaurant and it was a spectacular finale to our evening.
Well worth it! We loved it!