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.

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.

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?

I just looked on the website and didn’t see the evening direct flight there, but I did see that flight a few weeks ago. Does anyone know why this direct delta flight might be seen by some but not by me?
That flight would be perfect!

I picked a random date in June.

No idea if its seasonal or just select days of the week

EDIT
Turns out OP needs a flight in August, ( disclosed by OP post below) when the schedule is different than what is available in June.

I have had it take an hour for luggage to show up before I could go through passport control ar SEA more than once. I personally would want more time for this connection.

Delta is my go-to airline and SFO is my home airport.
I know Delta doesn't have direct flight from Heathrow to SFO, BUT it is now partnering with Virgin.
I usually use a computer/browser to look at and book flights. If you go on Delta website and search for your date, you can go to "filter" and unselect "1 stop". So you only have nonstop options. You will see all daily flights. Not sure if on the app you have the option to filter to show only nonstop.

I don't know if they have it daily, but I do see a couple of flights for the random date I checked. One is 12:45pm and another 6:30pm. I wasn't logged on when checking. The flight will have a Delta number but it is OPERATED by Virgin Atlantic.

And I have flown thru SEA a lot, especially leaving from SFO with connection at SEA on way to Asia or Europe. Usually not an issue that way since I'm already in the US.
I have arrived twice at SEA from abroad. Once I had a connection that was less than 2 hours but I also have Global Entry and I fly with carry-on only. So it was not an issue. The lines were actually short for regular passport control but I have no idea for bag. (I have lost connection at JFK once because my bag took so long).
The other time I had a huge layover (because my original flight from Tokyo was 3 hours delayed so I missed my connection at SEA).

Thanks so much everyone!
What I am seeing is that there is an evening non stop from HRW to SEA until Aug 1, and then there is only a 12:45 or so non stop after Aug 1.
I am wondering if they will add that non stop for after that date.
Another option would be United that has an afternoon non stop, but I have used Delta more so I was only looking at Delta to start this process.

You might try google flights and put in nonstop between London and SFO to see which airlines have nonstop flights for your date. Doing a random date, I see United, BA and Virgin Atlantic.

"there is an evening non stop from HRW to SEA"

Apologies since this is going to sound SO pedantic but.....

The 3 letter IATA code for London Heathrow is LHR

The 3 letter IATA code HRW is not assigned to any airport.
https://en.wikipedia.org/wiki/List_of_airports_by_IATA_airport_code:_H

I'm only mentioning this because if you are searching using HRW you are not going to turn up the flights you want from London.

What I am seeing is that there is an evening non stop from HRW to SEA
until Aug 1, and then there is only a 12:45 or so non stop after Aug
1. I am wondering if they will add that non stop for after that date.

It is unlikely they will add a second nonstop flight if they don't offer one now. Virgin Atlantic indicates there is a daily nonstop to and from Heathrow and SFO. It is entirely possible that Delta doesn't offer every single partner flight every single day. Have you tried booking via Virgin Atlantic? https://www.virginatlantic.com/en-US/where-we-fly/north-america/usa/san-francisco

Also, you might list the specific travel days you're trying to book to/from so that might clear up the confusion for what you see versus what others see. It's possible your search parameters are different.

We are trying to book Aug 3 from SFO to LHR then back on Aug 12, but in the evening on the 12th.

This is for our son who lives in SF and will join us on our trip.
We have a booking with Delta for him, refundable, that before was on an evening direct flight back, which I guess they canceled, and they moved us to the noonish direct flight. We would like to spend the morning of the 12th with him in London.

Sorry I got the code wrong. I have been rushing around this morning and was doubtful I had typed correctly but didn’t double check. I should have.

I don't see that evening flight on August 12 (LHR-SFO), reason they probably moved him to the earlier flight. Sorry.
I think if you want to stick to Delta, he would need to be on that earlier flight or the next morning if you want extra time.

But United does have direct flight both ways with the one departing London Heathrow at 4:15pm on Aug 12.

FWIW, a Google Flight search for those dates gives a British Air direct flight option both ways, outgoing flights at 4:35 p.m. and 7:35 p.m. and two direct return flight options of 10:45 a.m. or 2:20 p.m. Only United has a later departure direct flight at 4:30 p.m. If you're wanting to spend more time with your son, could you add a day earlier or later?

Thank you David and everyone for your replies.
I have not clicked on anything yet. Delta does not appear to have a later direct flight now, past Aug 1, so I could ask for a refund and get a United flight instead.
Our son only has so many days he can be gone on the trip, so we want to get him an evening flight as I said so we can have that extra half a day in London with him. The original flight was an evening one as I mentioned. So…maybe United.