What are some worthwhile attractions to look at in London?

Eveeywjere

Museums, British library, Buckingham Palace, London Eye.

All the major museums are great and FREE. Avoid expensive tourist traps like Madams Tussauds and London Dungeon. A river trip is worthwhile and surprisingly reasonable for London.

Novachok restaurant where Russian spy choked to death after microwaving his insides.

What do you consider 'worthwhile'?

There are some excellent galleries, museums and exhibitions in London. The Wellcome Collection is always worth a visit.

There are hundreds!
Do some research.