We will be travelling from the U.S. to Heathrow, then connecting on a Swissair flight to Zurich. Does anyone know if we actually go thru immigration and "enter" the UK or will we be kept "in transit" where we technically never enter the UK? Can we choose to go thru immigration if we want so maybe we can get outside a bit since we have a 5 hour layover and are travelling with 2 youngish kids (5 & 7).

You won't pass through UK immigration, you will pass through security & customs.

You can go through UK immigration but it will typically take an hour from landing to you exiting into Heathrow. Enough time for a quick trip to Windsor and not much else.

If you go to the Heathrow Airport website and do a search for "flight connections centre" the 2nd and subsequent answers tell you what to do depending on which terminals you are transferring between

No, you won't be kept...it's totally up to you if you want to stay at the airport or not. The only difference is that you'll have to go through security again when coming back

If just transiting, you'll go through a security check before going to your departure gate. Passport control and customs will be at ZRH (assuming your bags are checked through.)

If you go "landside" then I agree Windsor might be a good taxi trip - around 15 min. Show the kids Eton College so they can start planning (just the boys, sorry.)

If your bags aren't checked through (e.g. due to British Airways' reluctance) then you'll need to enter the UK and fetch them and schlep them to Terminal 2 for your <i>Swiss International</i> flight. (Swissair is no more.) In that case you'd need enough time that an excursion is impractical.

I live in Switzerland, we all still call it SwissAir - the new name is too long. It doesn't really sounds like you have much time to do much because once the plane lands it can be an hour or more to get out, get in and out of the security line and then you will need to be back at least an hour and a half (even though 2 is recommended) before the ZRH flight. That leaves at the most 2-1/2 hours. Not a whole lot of time to go sightseeing.