Arriving in London at heathrow, United flite .

Is this on one through ticket (one booking)? Assuming "YES":

• You bags will be labelled through to Paris, you will pick them up there and take them through customs.
• You will stay "airside" at Heathrow, not legally enter the UK. After you land in Paris you go through immigration (passport control) for entering the Schengen Aea.

If "NO", you will need to go through UK immigration, pick up your bag, go through UK customs, check in again (hand over bag) and exit the UK. This will take a lot longer.
And, if it is two different bookings, it is NOT a guaranteed connection in the event the first flight is late or you take a long time to go through immigration. In this case if you miss your second flight, hard luck.

Are you arriving and departing from the same terminal? if not, it could add ~1 hour.
Note immigration occurs BEFORE customs and BEFORE you pick up your bags. Immigration could be 10 minutes or 1 hour, customs takes no time.

Thank you. I may have messed up, as we are scheduled to arrive at 7a.m United in London, then British air at 10:55 for Paris. At beginning of planning trip we had not intended to go to Paris. Do you think 4 hours with a carry on only bag is enough time. Thx for your help.

If you only have carry-on, you will have enough time. You won't have to go through immigration or customs.

Go to this website: Heathrow Flight Connections, fill in your flight information and it will give you step by step instructions on what you have to do.

You will change terminals but it will be airside.

They arrive Terminal 2, depart from Terminal 5.

Really you should start your own thread so carrie doesn't get notifications.

If you have to check in a bag in London then you will have to go through customs and immigration and check it in landside. There is no facility to do it airside. BA possibly might do it at the gate however if they refuse it (but note it will still need to be collected at the carousel).

As to the OP, do check in online for your Paris flight the day before. Saves time and is easier to do the airside transfer already with the boarding card in your possession. You will go through security in Terminal 5, mixed in with people checking in there.

As Marco stated, you will have to go through immigration and customs to get your bag and check in for the BA flight. Fortunately, you won't have to change terminals.

If you haven't purchased the ticket, see what combination you can get with BA or AA (they book BA flights on their website (AA codeshares).

The OP on this thread only has carry on so can stay airside even though they are changing airlines and terminals.