As a self-respecting Flyertalker, I've visited London on <24-hour layovers at least a dozen times.

-Check your bag through to DXB
-Clear immigration and customs and then shower at the Arrivals Lounge if you have access (J, F, status)
-Leave your carryon with Excess Baggage, remembering to take your phone charger, etc., with you
-Use HEX or Tube or Elizabeth Line to Central London

London has many excellent museums, from the mainstream general audience places like the National Gallery and Natural History, to quirkier/special interest places like the Handel-Hendrix House, Foundling Museum, or Royal Institution.

London has many excellent restaurants, and while I've found a few I really like (Cicchetti for Italian food at Piccadilly Circus, Bombay Palace for Indian food near Hyde Park, Pick and Cheese at Covent Garden, etc.), others on here will know that scene better.

But for goodness' sake, the best thing about London is just to walk around. Walk through Kensington Garden and Hyde Park. Walk down (or up) the Thames. Walk through Mayfair and Marylebone to end up in Regents Park. Any of these, or others, will show you good scenery and a wide variety of humans; they will also help you with jet lag and exercise before your next flight, with the added benefit of being virtually free.

And along the way, if you're so inclined, check out the churches. As a Catholic, I'm a fan of the Brompton Oratory, Immaculate Conception at Farm Street, St. James in Spanish Place, Westminster Cathedral, Corpus Christi, and St. Etheldreda. Each is beautiful to behold, and each has a story. Again, these are free to enter and open most of the day.

These are my tuppence worth of suggestions. Enjoy one of the world's greatest cities!