what to see within 10 hours in London

What time do you arrive? Do you want to get a hotel in central London for one night? Or do you prefer an airport hotel and then go into London? Are you interested in just walking around? If you chose to stay near Victoria Station, a good route would be to take the tube to Westminster. At Westminster, check out the Abbey from the outside and the Parliament building, wander up Whitehall, look for both Old and New Scotland Yards, stop at the entrance to Horseguards parade and admire the mounted Guards, then up to Trafalgar Square, have lunch in the crypt of St Martin’s in the Fields, then continuing around Trafalgar Square past the National Gallery, through Admiralty Arch and down The Mall to Buckingham Palace. Then walk to Victoria Station, take the tube to Paddington and transfer to the Elizabeth line back to LHR. Don’t forget to give yourself plenty of time for that journey as things can happen especially toward rush hour.

Of course you need to figure out what to do with your luggage…..leave at airport and carry minimal needs for one night and carry that around for the day?

We will arrive Heathrow Airport (LHR) at 20:30pm and stay a night near Heathrow Airport. We like to go to central London next day before we are going back to Heathrow Airport around 19:00. Which public transportation do you recommend traveling between Heathrow Airport and central London and what to see? Thanks

If I were in your shoes, I would choose two of the following sights and allow 3-4 hours at each: Tower of London, Westminster Abbey, British Museum. I would not waste time on the Changing of the Guard or Hop On Hop Off buses. I would want to do a somewhat deep dive into the two sights that interested me the most. We always take the Heathrow Express to Paddington, but have not tried the Elizabeth Line and since you will not be encumbered with luggage, there may be other choices that would be easier for your sights. You might want to make that decision first and then ask the best way to get to the first one from Heathrow.

Its all personal preference, isn't it. For me, I'm like cala, I would go to the British Museum, and one other central site that interests you. For me, the second choice would be either Imperial War Museum or National Gallery.

Be careful which hotel you choose to stay in. The hotels along Bath Road might be cheaper but it will make the journey into London more awkward with a bus ride needed to get back to the airport for a train into town. Doubly inconvenient if you need to go back their luggage at the end of the day. Try and get a hotel at the terminal you leave from.

Personally I would stay in London if you want to see somewhere - otherwise you are using up an hour of the visiting day for travel in.

Either way I would look to go to one place and do it well - so Tower of London, Westminster Abbey type places - and easy to use the Piccadilly line to Baron's Court then District line from there.

Or else pick a free place so you can bail if needed - Piccadilly line to Russell Square for British Museum as an example

Its the city center not downtown. You say downtown and no one will have a clue.

Sites:
Parliament Square ( Parliament and Westminster Abbey)
Westminster Bridge
Churchill War Rooms
St James Park; Birdcage Walk up to the Palace
Walk up Whitehall to Trafalgar Square
National Gallery if it’s reopened
Leicester Square then over to Covent Garden
Harrod’s Food Halls
Chinatown

Its the city center not downtown.

I've always heard it referred to as "Central London".

National Gallery if it’s reopened

It is. Just spent 3 hours there today and may need to go back to finish it up.