London 2027

There will be little or no effect on tourist facilities over the Easter weekend. Pretty much everything will be open. It’s likely that major religious sites like St Paul’s and Westminster Abbey will be closed for touring on the Sunday. Also by law all larger shops will be closed on Easter Sunday.

Finally if you were planning to get to your cruise by train (usually the best way), you will need to check schedules nearer the time as Easter is a popular time for line closures for maintenance.

I wouldn't recommend booking an organized tour in case your flight is delayed. You might consider one of RS's walking tours on the RS AudioEurope app (Westminster or Historic London). https://www.ricksteves.com/watch-read-listen/audio/audio-tours/london
Re: what's open and what's not, I suggest making a list of the sights that are of interest and check their websites. I just checked, and the Tower of London appears to be open both days.
Finally, there is a typo in your post. Your arrival is 3/28/27, not 8/28/27.

London Walks are fun and they have a number of them going on Easter Sunday and Monday. https://www.walks.com/our-walks/walks-by-date/?d=2027-03-28

This website (the official London tourist site) should be helpful as well: https://www.visitlondon.com/things-to-do/whats-on/easter

Any ideas on what tours we can take that morning or what we should see that will be open?

What do you want to see?

Besides cruising what are your interests?

Art? Food? History? Architecture? Museums? Shopping? Theatre?

“Cruising”? (Not judging).

It would be good to know your interests.

Easter is 3/28/2027. Not 8/28.

I’m planning 3 nights in London for mid-April after a tour of Devon and Cornwall. I’m looking forward to it!