I have a very quick trip to London. I will arrive at Heathrow on Sunday 9/9 around Noon via Iceland Air. I need to get to Trafalgar Square (presumably by train/tube).

On 9/10, I will only take the tube twice (Trafalgar to Oxford Circus and then return at the end of the day).

Then on 9/11, I need to get to Gatwick on Tuesday, 9/11 for a flight at 1:00 for return to the US (also on Iceland). Again, I presume that there is some form of rail transportation that is efficient.

Any tips would be appreciated.

Take the tube in from LHR. Do you mean Exactly Trafalgar Sq -- or somewhere in the vicinity. That could make a difference re what station to use. But in general, to get to Trafalgar Sq, you take the Tube from LHR to Piccadilly Circus and walk to Trafalgar Sq . . . Or take the tube to Hammersmith, walk across the platform and jump on the next district line train, get off at Embankment and walk to Trafalgar Sq. Which is better depends on exactly where you are headed.

Then take a (regular) train to LGW from Victoria to LGW.

Trafalgar Sq could be walkable to Oxford Circus - again depending on exactly where you will be. From say the National Galley to Oxford Circus is about 3/4 of a mile and less than a 20 minute walk.

Agree ^^^^^.

Also from Tralfgar Sq could take train from Charing Cross station just across street and with one change at London Bridge station be at Gatwick in about 40 minutes - may be easier than taking Tube to Victoria Station and those trains via London Bridge are I believe cheaper than Gatwick Express. Seems easier all in all than schlepping over to Victoria.

The local train from Victoria to LGW is run by Southern. Usually a few pounds cheaper and a few minutes longer than the Gatwick Express.

We used to always take it back when USAir flew into Gatwick and we stayed near Victoria Station.

ssander

We don't know where exactly the OP will be -- not likely in the middle of Trafalgar Square sleeping with the Lions/Admiral so whether Charing Cross to London Bridge, or Victoria, or other options make sense we do not know.

We need the OP to respond before we can make specific suggestions.

My post was re Pal's post, didn't see ssander's. Agreed -- no, need for the pricier GEX.

Southern has had a lot of disruptions to its service this year.