10 hours in London isn't a lot of time. Especially if it takes you, on the low end, 60 minutes to deplane, clear passport control, and so forth. Then another hour into Central London. By the time you factor in the return travel time and what not, maybe you have 6 hours in London. I'd rather go to Windsor, which is much closer to Heathrow. Maybe 15 minutes by Uber.

I've done the 20-23 hours in Amsterdam before. I find Delta.com regularly gives me that option on revenue tickets when clicking through from Google Flights or Book with Matrix.