Connecting flight question..

I would check what terminal you are arriving in then, head to Heathrow's website and review the online info about getting around the airport. You likely/may arrive Terminal 5 and depart Terminal 5 and yes it is large and transferring by foot once deplaning and getting to the next gate will be a large airport stroll experience with all your carryon paraphernalia. Lots of signage and taking the time to orient yourself in the terminal by using an interactive map to move around can help allay concerns and questions. Whenever this trip is slated, bon voyage!

This link can give you information specific to your flights
https://www.heathrow.com/connecting-flights

When? There is a strike scheduled at Heathrow by security employees at the end of this month. Best to Google and have your own direct info.
Safe travels!

https://www.bbc.com/news/uk-64994967

Your San Francisco flight will land at T5 and the flight to Lisbon should depart from the same. At least I do not see Lisbon on the list of airports used by BA from T3.

https://www.flyertalk.com/forum/british-airways-executive-club/1838106-list-ba-flights-t3.html

Note that T5 itself has 3 separated buildings, connected by an underground train that runs frequently. They are T5A, T5B, and T5C. Our flight from Heathrow to SFO yesterday left from Gate 55 in T5C, but that could change.
If you land at B or C, you follow the Flight Connections signs which will take you to T5A (using the train) , where you must go through security again, but you do not go through immigration (passport control) or customs.

Security at Heathrow is very rigorous. Make sure your liquids are properly packed, and you take the liquid bag as well as your iPad, laptop or tablet out of your luggage. Watch what other people do as you wait to be assigned to a station to load your luggage and other items into bins for screening. You will take a bin from underneath the station, put it on the rack, and load it. Then you must push it into the farther lane with the rollers for movement into the scanner. If you need another bin for liquids, laptop, jacket, etc., load that and push it into the far lane before you approach the scanner. We have seen lots of people who were unaware they had to move their bins onto the rollers themselves, and walked thru the scanner only to wait and wait for their bin that never arrived. They finally figure it out and have to go back through to push the bin onto the rollers.

Count yourself lucky if the bin with your bag or other items doesn’t get kicked into the “secondary inspection” lane for further scrutiny. Mine did on our trip to Rome a month ago; it took about 20 minutes to go through the secondary inspection process. The items that aroused suspicion were a small Venice guidebook, my sunglasses in a case, and a metal insulated mug. These were removed and taken over to be swabbed and re-inspected for explosives residues ( at least I assume that is what it was all about).

For our flight home yesterday, as was waiting for the bin with my little daypack to come out of the scanner, I watched almost half of the bins get kicked into the inspection lane. Fortunately, mine did not, nor did my husband’s.

Once through security, you enter the large shopping/waiting area and watch the boards for your flight and gate to be posted. If it is in B or C, (the higher gate numbers), you go back on the underground train to that terminal. Again, it is well-signed. Sometimes they do not post the gate until 30 minutes ahead of the flight. When they do post it, proceed directly to the assigned gate.

One bit of warning—-if you are on BA 286, the A380, they have been having a lot of “technical” issues with those planes that cause delays. Our flight was 4 hours late leaving Heathrow because of that. If you are on BA 284 you will have a 787.

Heathrow is a challenge! But, with the length of your layover, you should be fine. I’ve gone through Heathrow a number of times and find that I need about two hours. I have had times when I didn’t have to change terminals but most of the time I have. The bus service is good and everyone else is doing the same thing so hard to get lost. The good news is the shopping at Heathrow is good!

That should be plenty of time. I believe the Heathrow connections website will tell you you’d need 1.5 hours to get from T5 to T3 if that’s where you need to go. I believe however you’ll land and depart from T5. If that’s the case you’ll need an hour to get to the new gate. You will need to pass through security again. Just follow the purple arrows/lines for connecting flights upon arrivals. That line will split with one way for within U.K. flights and the other way for other destinations.

You will not go through immigration at LHR. All you need do is make your way to your connecting flight passing though security checks on the way.