One day in London

Rick Steves has a template for a London itinerary for 7 days. I suggest you review that here and use it to cobble together something that works for you.

Simple. Do Parliament Square. Don't be too disappointed that Elizabeh's Tower what is world renown as Big Ben ( which is the bell not the tower) is encased in scaffolding. Much needed repairs but you'll still see Parliament and then can tour the adjacent Westminster Abbey. After walk up Birdcage Walk through St James Park to see Buckhingham Palace. Then walk through Green Park Over to Fortum and Mason for afternoon tea... Then...off the Gatwick.

Great city. Enjoy your few hours in it...

Here's what I have planned for our six hours in London. (We have been there for longer stays several times)

We will take the Heathrow Express to Paddington mid-afternoon, check into our hotel, then take a cab to a start point near the London Hilton Green Park,. We will walk through the parks, past the Abbey and Big Ben, cross the Thames and walk along the south bank to the pedestrian bridge, then make our way to the Sherlock Holmes pub for an early dinner, Trafalgar Square and Picadilly Circus, then pick up a cab back to our hotel adjacent to Paddington Station. Plenty of places on the south bank to stop for a sit-down or a drink. Next morning, return to Heathrow for a late morning flight to Geneva. (Convoluted flight schedule because we can fly free on points SEA to LHR) The plan is to make the best use of our time and enjoy seeing some of our favorite London spots. I was not trying to be budget conscious for this day.

The walk is 3.5 miles, You can shorten it to 3.0 miles if you start in front of Buckingham Palace instead.

Here's the map, if it works....
https://www.google.com/maps/dir/Hilton+London+Green+Park/Westminster+Abbey,+Deans+Yd,+London,+UK/Big+Ben,+London,+UK/London+Eye,+London,+UK/Hungerford+Bridge+and+Golden+Jubilee+Bridges/Sherlock+Holmes/Piccadilly+Circus,+London+W1D+7ET,+UK/Trafalgar+Square+(Stop+W),+London,+UK/Charing+Cross+Station,+London,+UK/@51.5047857,-0.1407889,15z/data=!3m1!4b1!4m56!4m55!1m5!1m1!1s0x48760528dd211a43:0x2e4686ea0a35c24c!2m2!1d-0.1456146!2d51.5064534!1m5!1m1!1s0x487604c359914745:0x700da0b0ccd9ee00!2m2!1d-0.1275705!2d51.4994174!1m5!1m1!1s0x487604c38c8cd1d9:0xb78f2474b9a45aa9!2m2!1d-0.1246254!2d51.5007292!1m5!1m1!1s0x487604c71a3d651f:0x72367f97712bc0ee!2m2!1d-0.1195323!2d51.5009095!1m5!1m1!1s0x487604b7f8580f1f:0x31b07fc6b90b3bc5!2m2!1d-0.120054!2d51.506136!1m5!1m1!1s0x487604cee398589d:0x12fa2d3037afe846!2m2!1d-0.125297!2d51.5073474!1m5!1m1!1s0x487604d3ff201fc1:0xf08adf0cfb3eb2fe!2m2!1d-0.1345676!2d51.5100913!1m5!1m1!1s0x487604ce538ec6bd:0xb4d7bc36a54a6e23!2m2!1d-0.1264738!2d51.5073326!1m5!1m1!1s0x487604ceeaadbb11:0xdf61c9ec353e1637!2m2!1d-0.1248116!2d51.5081101!3e2

How many hours do you have in London? If you have several hours (5-6), you can do one of the walks suggested by others, plus one big sight, such as Westminster Abbey or the Tower of London.

If you have 7-8 hours, you could see two sights, take a Hop-On-Hop-Off bus tour, plus do one of the walks described by the previous posters.

We flew into Gatwick about a month ago.. arrived at 12,30.. got luggage checked into our airport hotel, by about 2.

We took the Gatwick express in to city.. then we walked our butts off.. we walked by Buckingham Palace..down the Mall, visited Trafaglar square ( sorry I think I spelt that wrong) .. then from there to Picadiccly Circus.. where we saw a movie premiere and MET Russell Brand.. lol .. then had dinner and took train back to hotel.

Saw palace ( outside) , Big Ben ( down the street) Trafaglar Square, and Piccadilly Circus.. ( which honestly is just a bunch of restaurants and a lot of atmosphere.. lol

What I would have done differently is I would have gone to a museum.. I love them ( and ive been to most of the big ones in London several times already) but friend I was with hates them.. ( boo) .

I would go into London.. and visited the Victoria and Albert or Imperial War Museum.. or Churchill War Rooms.. then a pub meal and a walk about.. that would have been my ideal short visit.

A lot depends on what you want to see.. with 6-8 hours you can visit one major site and have a nice meal and walk about. If not musuem person ,,, and you have at least 7 hours, I would try Tower of London.. ( friend is not into history either.. sigh) .

My favorite is the Tower of London, but for me that was probably a four or five hour visit. The time just flies there. The other one would be the British Museum, where you can have a very good visit in three hours, or even two. Westminster Abbey is pricey but a great visit, probably two or three hours. The National Gallery is great but not as unique to London as the other two. I mean the paintings in it are great, but many large cities in Europe have great art museums. The portrait Gallery next door is also great, and you can have a good visit in an hour and a half (if you don't want to spend too much time in museums). Another museum that I liked on two separate visits was the Churchill War rooms. Most people would probably spend less time than I describe at these sights, I like to linger. I would pick one of the above and spend the rest of the time walking around. Rick has a really good walk in his England book, from Westminster Bridge to Trafalgar Square. He also suggests some routes on the red double decker busses, many of which stop at Victoria station.