Gatwick to Heathrow?

The actual drive to Heathrow takes abt 50 mins, so Uber? There are transfer Coaches at several terminals, but you would have to get your luggage aboard in a timely manner. Even if you succeed to get on the Coach; they do advise that you arrive at Heathrow 3 hrs ahead. See if you can get a flight at a later time to a West Coast City with a connecting local flight, bus or train. Worst case is that you could drive a rental car to home base. I missed a flight to Seattle, so I flew into Portland. It was only a 3 hr drive, so my Mom came and picked me up. Try getting a flight from Gatwick first. If you have flight mileage points on your cards, try to use a Canadian or European Airline as they are said to give more mileage when you redeem them. Good Luck!

The problem is that if your first flight is delayed and you miss your second flight, you will need to purchase a new ticket for the LHR to SFO and that could be expensive. It is not a risk I would be willing to take. I have bought separate tickets, but I always add an overnight between the two to minimize the risk of a delay of a flight cancellation.

If you attempt this, it is very tight. Your best bet would be:
1) Use carry-on luggage only. When I arrived at Gatwick with a checked bag in 2022, I waited about 45 minutes for my bag to show up on the carousel.
2) I’d book a car service between the two airports. The drive is about an hour. There is a coach, but unless things are well timed, you could have to wait up to 30 minutes for it.

3) Check your travel insurance to see what it covers if you miss your flight.

That is very risky. You are meant to check in at LHR for a long haul flight 3 hours early- 1.15 pm.

Even in a taxi LGW to LHR is 55 minutes if there are no issues on the M25. Train or bus will take too long. Even if you got straight on a bus it takes 1 hour 25 minutes. A train is 1 hour 30 on very, very optimal timings, closer to 2 hours on realistic timings. Plus time to clear immigration and the airport. So taxi is the best bet, but you will still arrive at LHR significantly under 3 hours early.

And carry on only, no checked bags.

Add to that the fact that flight tracking (on Flightaware) has EZY 8596 arriving 18 minutes late today (8 September), 67 minutes late on Friday 6 September and yesterday. But to be fair on time last Sunday and 6 minutes early on Sunday 25 August.

There is a BA/Croatian flight to LHR from Split changing at Zagreb, arriving at Heathrow at 1220. Today that flight was cancelled from Zagreb, last Sunday it was 19 minutes early.

To save some money over taxis, you might want to look into a private car service like Blackberry Cars or Just Airports.

one bit of good news is that the full weekend closures of the M25 at Junction 10 for the A3 have finished for this year (2 more next year) so the main motorway between Gatwick and Heathrow won't be completely closed so no huge diversion.

There are still 12 hour closures while they work on the junction, and without your exact date of travel we can't tell you if you might be affected.

If so, it would make a very tight plan even tighter.

Since the flights are not related, if you miss the flight home you will be hoping your travel insurance will help you buy a new ticket

I think the first thing is to check your insurance to see if it will cover delays. If so, you could stick with your current arrangements and hope that things go more or less to plan (although I think I'd book a car from Gatwick to Heathrow to avoid waiting for a bus).

If not, I think the best plan would be to travel to the UK a day earlier or back to the US a day later. There are just too many ways that you could miss the flight to SFO.

By the way following up on the cancellation of the flight from Zagreb yesterday news stories this morning are saying that several dozen short haul flights into LHR and LGW were cancelled yesterday due to a combination of thunderstorms and air traffic controller shortages. But UA939, your flight, actually took off 6 minutes early!! So swings and roundabouts.

And a few days ago most flights into London City were diverted to the usually sleepy London Southend airport due to poor visibility.

It is just a general be aware note when booking non ticketed connections of this type.

Yes SOU to London is easy from the adjacent railway station, but it was added time for those affected.

There are buses, but if it were me, I would do anything in my power to have everything going from one airport. Yes I know you probably already thought of that!