SEA- is this long enough to get to a second flight?

Single ticket ?

Even with a single ticket, this will be very tight because you will have to go through TSA checks again. TSA Pre lines are very long at SEA.

Maybe, but it would be cutting it rather close. What time of year? Time of day and day of week? Do you have Global Entry (when operational)? SEA international arrivals can be a nightmare and it can be totally fast - I've experienced both. You have to collect checked bags before going through passport control, then recheck to SFO after that. The SEA page with the process says this can take 60 minutes if it's busy: https://www.portseattle.org/services-amenities/international-arrivals-and-passport-control

On the up side, you'll likely have a later SFO flight option if you miss the first one, so long as you're on a single ticket the airline would be obligated to help you get home.

What airline? What time?

If you miss the connection what’s your next option to get to SFO?

It would be a single ticket. It is the last flight of the evening going to SFO from SEA. It would be Delta.
It might be too tight for my comfort but it would give us more time in London.

In 2024, due to a very long delay in AMS we had about that much time to make our SEA>LAX. I actually reached out to the forum while sitting on the plane in AMS asking if we should try to change our connector or if we had a prayer of making our connection. Changing flights would have required us getting a hotel in SEA and a flight out the next morning and I really was just ready to be home. We were sitting 2 rows behind business in Delta Premium Select; had checked luggage (supposedly had Delta priority of luggage coming off the plane first..true in theory NOT in practice!) We were already prepared with our Mobile Passport App. As soon as the plane touched down in SEA and still on the tarmac we took the selfies and uploaded the required answers into our app and hit submit. We took the tallest escalator ever - (confirming that the responses on the forum knew SEATAC.) We were directed to the luggage belt and waited for our luggage which due to Murphy's Law meant ours was about last to arrive. Then we got in the designated Mobile Passport App line vs the regular line. There wouldn't be a chance if we were waiting in the regular line. The only people with more priority than us had Global Entry. The friendly officer looking at my passport read my stressed face, welcomed me home and said to get going because I told her we'd been delayed a few hours! Once past that, we had to drop our checked luggage back on a belt. Then...omgosh, I didn't see this coming...because I thought we were close...then we had to get in one line to go through TSA. No TSA precheck...every shoe, (I think that's since changed?) every belt, every computer out. The guys in front of us kept having to pull things out of bags, and repeatedly go through the scanner. Meanwhile, I tell my husband we do not have time to make a pitstop...get to the gate and see if we have time. We look at the sign for our gate not knowing the layout of the airport. We jog/run with lungs burning (I disclose because I don't want anyone thinking we run under ordinary conditions.) We are indeed assigned to the furthest gate of the furthest concourse. We arrive at the gate as they are seconds from boarding; when they suddenly announce the computers are down and they can't scan the phone apps! We made it! So the moral of the story is get in shape, carry on luggage only, print out a ticket as back up (Delta in AMS did even though we didn't ask them to do so!) maybe you'll fine, maybe you won't...I'm not sure!

I see Delta sells a Virgin Atlantic direct to SFO flight leaving LHR at 630PM ( leaves 45 minutes later than the SEA flight) and gets to SFO 2+ hours earlier.

Is that under consideration? For the random date I chose its actually cheaper than the flight through SEA.

Thank you Joe! I don’t see that flight. How can I find that one on the app?

Tight, you’ve got a bit of a walk to get to and go thru customs then TSA and will probably also need to take the airport train to another terminal. But if it’s all on one ticket and you miss the connection they will get you on another. There’s a lot of Seattle to SFO flights.

Update. Just saw “last flight of evening.” Hmmmmm, why not London to SFO?