Bank holiday - what's open and what's closed?

You will want to check planned works on the tube as there tend to be closures on bank holiday weekends.
https://tfl.gov.uk/status-updates/planned-works-calendar

I’ve noticed that museums and sites that appeal to kids tend to have longer lines.

In London, other than the banks, some small independent businesses such as cafes maybe closed over the bank holiday. Most of the major shops and attractions will be open, but check the website of a particular site if you are planning on visiting somewhere in particular.

People taking advantage of the long weekend and the end of the summer break for schools means places will be busier than normal.

There are often engineering works on rail and tube networks - check nearer the time for details.

That’s a great question! We planned on going to Windsor, guess I better check train schedules!!

remember that there are often ways around. For example, trains from Paddington and Waterloo both serve Windsor and Eton, on different routes and into different Windsor and Eton stations. Unlikely both affected. So you may be a couple of Pounds poorer or richer, and it may take a few more or a few fewer minutes, but you'll probably get there....

August Bank Holiday weekend is also Notting Hill Carnival weekend, so that area of west London will be very busy on the Sat and Sun. Better check their website so you can see what’s going on

You're right AmandaR, I'd forgotten that the mass gathering is back on. With covid out of everybody's mind and desperate to make up for lost time I expect it to be even busier and madder than ever.... Well warned...