BA connecting flight at Heathrow - which one to choose?

You will not have to go through immigration if you are buying one ticket. You will go through a security check, but you will stay airside.

The Heathrow connections guide will show you the steps you will follow and which terminals are involved.
http://www.heathrow.com/flight-connections

If both flights are BA, then you don't pick up your checked bag until Barcelona. All my BA flights from Boston, and connected, also flew out of terminal 5. However, you can go into BA.com and check the terminals. When both in terminal 5, I usually get through passport control and security, by following the purple signs in about one hour. However, I usually have my layover time as 2 1/2 to 3 hours as I'm conservative. That being said, if it is all on the same ticket then it is BA's job to get you onto the next available flight, and as there are two after the 105 minute layover, you should be pretty safe doing the first one as it is a legal layover time. If you have to change terminals, then you should rethink this.

If it is all on BA take the 1 hr 45 mins.
If you missed that one BA will put you on the 2 hr 45min one and so on.
So there is really no need worry about the connecting flight.

Great information! Thank you all for the prompt responses. Sounds like I can safely do the 1hr 45 min connection, since there are backups available after that flight.

Barbara

I have done connections through Heathrow a bunch of times with 90 minutes or less going to France, Spain and Italy. We are a family of four and we have never even come close to missing our connecting flight but we are batting around .333 for our luggage making the connection. Each time BA has delivered our luggage to our hotel by the end of the day.

JHK, we've had our baggage misplaced/delayed with EasyJet, Vueling (twice!), and American, so we're not doing much better! Fortunately though, we are staying with family in Barcelona so if the bag arrives a little late, it won't be disastrous.

Pick the connection in which you can stay in the same terminal. If that's the case for all, I'd pick the one with the 2:45 connection time to allow for your inbound flight to be late.

Keep in mind, if you miss a connection, if the next flight is booked or overbooked, you won't get a seat. This is why I would pick the 2nd itinerary with the 2:45 connection time.

I like Continental's thinking. Gives your bags a better chance to make the connection and you have time between flights to use a real bathroom!

Absolutely go for the 2 hour 45 minutes connection. If your Boston flight is late it gives you a bit of a cushion. The reassurances regarding getting you on a later flight are not sound. It depends on how full those flights are and your status. If you are a BA frequent flyer someone may be bounced so you can take their seat. Otherwise better hope for an undersold flight, less and less common these days.

Barbara,
Another vote for the 2 hour and 45 minute connection.

I recently flew to Milan, with a connection at LHR, and was randomly selected for an extensive security check which lasted around 40 minutes. My small backpack and carry-on luggage was inspected by literally emptying most of my belongings out, so you will definitely want the extra time, just in case.
Note that security at LHR expects all liquids to be in your 3-1-1 quart sized bag, including your liquid medications (I travel with prescription eye and ear drops).

Keep in mind that you will most likely need to get to a different terminal and that takes extra time too.

Have a wonderful trip!

I did a quick check and BA fights from Heathrow to Barcelona leave from Terminal 3 and not Terminal 5. They suggest a minimum 1:30 to make the connection assuming everything is running on time. Personally, I think 1:45 is cutting it close.

Thanks everyone for all the info and advice. Since we do have to change terminals (thanks, Frank, for looking that up), I would agree that the shorter connection time will be cutting it too close. So given that we have to change terminals, then I assume we will have to go through customs/immigration and security again?

This will be a middle of the night connection, I hope that no other big planes come through at the same time but they certainly could. We once went through Boston immigration when 3 large planes had emptied at once. It took over an hour to get through, and airport personnel were passing out bottles of water to everyone waiting. For our next overseas trip, we'll have Global Entry, but of course that won't help us until we get back to the States.