My family of 4 will be in London over Easter Sunday. I assume the cathedrals will be busy with services so we won't go there that day (nor Good Friday) but what else can we expect to be closed or not visitable on Easter? Also, are there places we should avoid because they will be extra-crowded due to Easter? Thanks!

Shops will be closed on Easter Sunday by law (except small corner shops, souvenir stalls etc).
Most museums keep Sunday or Bank Holiday hours.
Restaurants, pubs and cafes open as usual.

As Alec says, shops over 3,000 sq ft can't open.

It's the longest holiday weekend of the year, so many Londoners go away, and the largest single group of tourists (nearby Europeans) typically don't come here as much at Easter as they do when the shops are open.

So London is generally quieter than normal. The immediately surrounding countryside isn't: there's more on at places like Blenheim and Warwick, but a lot more other people there. Not the best time for exploring the Cotswolds.

I take it you aren't religious, or at least aren't Christian? If you are Christian, why don't you observe Easter while there and enjoy the differences...Why not attend a church service, have a nice lunch and then spend the day at a park, weather permitting?

We went to St. Paul's for the 11:00 am service and then to the Tower of London (which was pretty crowded).

Thanks, everyone!