Research the Days Out 2 for 1 guide.
https://www.daysoutguide.co.uk/2for1-london
Over countless visits to London, at times, it’s been beneficial.
You’ll need to make a list of your “ must sees” and compare to what might be available via the 2 for 1 deal.
To obtain the pass you need to have ridden on a train. I’ve gotten the deal by traveling on a train from London to Bletchley Park. The same for taking a train to Hampton Court from Waterloo Station. Also did it by taking a train to visit the historic Dockyards in Portsmouth. After each train trip went to manned ticket windows at Waterloo and Paddington Train stations and received 2 for 1 booklets.
As mentioned by Nigel the majority of London’s museums are free.