If you are looking for a nice and romantic evening program in London, then why not go for an evening cruise at Thames? You can of course do it during day time as well, but it is much more magical and nice as the sun has gone down, and the illuminated bridges and buildings can be seen.
There are river cruises and which you can eat delicious food, while others are simply for you to enjoy the view. The prices are varying a lot, depending on what kind of cruise you are looking for.
If you would like some kind of cruise on the River Thames, we warmly recommend the following site: River Cruises in London at Viator