Will anything be open????

yes

and VERY crowded at that . . . . .

Almost all museums, churches, larger stores (incl Harrods); restaurants, pubs (except most in City of London).
Closed: banks, some smaller shops, specialist shops.
Transport: Bank Holiday timetable.

Very much like today:

Boutiques tend to be shut, as do other smaller shops. The stores will be open. Most of the restaurants are also open.

On a separate note (although I doubt it concerns anyone here): There is no congestion charge nor any fee for parking, and you can also park on single yellow lines.