Suggestions on what to see in London on an overnight layover. Arriving London at 3:20 pm (Heathrow), staying at hotel near airport and departing next morning at 10:15 am. Looking for recommendations of what we must see while where there and possible restaurant recommendations.
Thank you in advance!

You don't really have time to see anything. You won't get into London until after 5 and you'll have to leave before anything is open in the morning.

Basically you have one evening and night. Maybe you could go to the theatre.

You might look into seeing the changing of the guard. I believe it happens at 7pm. Last I knew you had to reach out to the Queen's secretary for tickets but something cool to do.

What sort of restaurant are you looking for?

You could get to Richmond and there are plenty of restaurants, not too far from LHR and you can get there and back by bus.

Or go to Windsor.

If you like Indian food, you are not far from Southall and a myriad of restaurants

There's a nice pub called the Ostrich in Colnbrook which is very close to Heathrow, need a taxi there and back.

Or go to dinner in London, take the tube and have a meal somewhere, the Wolseley is always a good bet, but there are 100s of places to choose from. Do you have a cuisine in mind?

Dinner and/or a show is about it really UNLESS it happens to be a Friday when some museum stay open late

British Museum is open til 8:30 (last entry 7:30) on Fridays, V&A is open til 10PM, National Gallery til 9PM

Each of the above also have on-site restaurants

If it isn't on a friday -- what sorts of food do you like and what sort of budget? Do you want a 'destination'-type restaurant or just a decent meal?

Which Guard Change is that? I have attended special evening tours of Buckingham Palace (very limited and must be booked pretty far in advance) but there was no Guard Change involved . . .

The 'normal' Changing of the Guard is at 11AM.

There is the Ceremony of the Keys, held at 9.30pm at The Tower. It goes for 30 minutes and you have to book ahead. Numbers are limited and rules are quite strict, such as no talking during the ceremony and no photography. I think we had to show ID with our tickets. It was fairly short but we enjoyed it. It's not put on just for tourists and is taken very seriously. The small group who have tickets are ushered inside The Tower grounds together and, at the end, let out again together.

https://www.hrp.org.uk/tower-of-lond...eys/#gs.qsneny

That is the Ceremony of the Keys - Nothing at all to do with a Guard Change which is what andrewboehmer4918 mentioned and I responded to. If that IS what andrew meant, it also has nothing to do with the Queen's King's Secretary. It is the locking up of the Tower. I've attended probably 8 or 10 times over 25 years. Tickets are now very hard to get. In the old days it was a complicated snail mail system -- but you were almost sure to get tix if you had any flexibility re dates. Now, being on line makes it really REALLY tough since people jump on them the minute tickets become available.

Oh -- re-reading my post -- I wasn't disputing anything KayF posted. But just trying to clarify the if that is what andrewboehmer meant - its not the same thing.

Welcome to Fodors.
More to consider:
When is the trip? In March or April (early sunset by 5:30, will make outdoor sightseeing more limited), or later in Summer (light to as late as 9:30pm in July when you could see more out walking).

What day of the week? Janisj already gave late options for some days.

Who is traveling? Children, teens, young adults, etc?
After a flight, would they have the stamina for a museum, or Interest in theater or concert?

What are current REC’s for time to arrive at the airport the next day?
Is luggage checked through or do you have to re-check bags?

Honestly, asking what you “must” see in London on an overnight layover like that is, sorry to say, moot. It is more a question of can you actually see anything? I can think of a dozen things I think are “must sees” in London. You might be able to do one. Which one depends on all the answers to above questions.
You will need to decide where you are going and precisely the best way to get there, no time to work anything out as you go.

Wish I could say take a bus tour (which I generally hate, but this is a special case) or driving tour just to get a glimpse of the great city of London, but traffic can be horrible, and it may be dark, so I don’t know. Maybe someone else has an opinion.

Today, 1st March, sunset is at 5.41pm. By the end of March sunset will be around 6.20pm. The clocks change to summertime end of the month & sunset will be at 7.31pm from April. And it does not get dark straight away like the tropics, it takes about 30mins to get dark completely.

In July, sunset can be as late as 9.20pm but light until 10.10pm.

I don’t see the point of presenting this as an obstacle to seeing anything of London, it’s like saying don’t bother going to Manhattan at night as it’s dark & you won’t see anything.

I did not say the time of sunset was an obstacle to seeing something of London, certainly did not tell them not to go, simply to note it as a consideration in choosing what to see. I have toured gardens, taken canal boat rides, traveled on the Thames to various places, gone to the big outdoor and flea markets in London, and there is not much to see in those places at night, just as some of Central Park in Manhattan isn’t great to visit at night. Of course some places in Manhattan are fun at night, some even fantastic at night, but some are definitely not, and it is reasonable to keep that in mind in any city. Some parts of many cities are beautiful at night. Certainly Central Paris is. All I wanted the OP to take note of is that, with their limited time and possible late time of day (which we actually do not know yet), they would need to be selective about what they attempt in the few hours they will have.
I even thought about the HOHO bus so they could see major buildings like Westminster, Parliament, etc in London at night. My personal experience with that has not been good, but others with different experiences might say they think it is a great intro to London.

The last time we stayed overnight at Heathrow our hotel had menus from local restaurants that delivered food. We had a delicious Indian meal without the need to leave the hotel. and so had plenty of time to get ready to fly home the following day.

You can stay right at the airport. We stayed at the Sofitel at Terminal 5 which made departing in the morning a breeze.