1 1/2 hour layover in Heathrow

As Emma says, no meaningful answer can be given without full detail of flights and destination. However, you won't need to go through immigration and customs unless your onward flight is to somewhere else in the UK or the Republic of Ireland.

Hello Susan,
I had a flight to Heathrow with a connection to Lisbon, had 90 minutes. I didn't plan it this way, the airlines had changed my flight plans about 2 weeks prior to departure.
I only had a carry-on with me. They had an additional security to go through anda spot to check your ticket too. I ran to the flight, they were holding the gate for myself and 2 other people. The plane started to taxi about 5 after I sat down into my seat.
I would have rather waited several hours than to go through all the running snd stress.
Hoping you don't have any problems with your connection.

The Heathrow website has a layover link that will tell you if your time is reasonable or not. 90 minutes is usually ok but tight unless it is not on the same flight code. If you are taking a flight with a different airline on a separate ticket, then any delay would likely void your connecting flight ticket.

For a connection to a flight outside the UK, you will not need to pass through immigration, but probably need to go through security if you change terminals. Security lines can be long.

IF you have carry one bags only and are connecting to a flight on the same ticket, you do not have to go through passport control ( or customs). You connect " airside" inside the security zone, even if you transfer terminals. You will go through security at the other terminal.

90 minutes is a legal connect time for most flights but it would worry me a bit, in case the overseas flight is late arriving.

IF everything goes perfectly, 90 minutes might work. But things never seem to go perfectly at LHR!

Allow time for a possible late arrival if anything gets delayed anywhere. 90 minutes is nothing when it comes to possible international delays.

You have to get off your plane when you arrive. Depending on where you are sitting on the plane and what size plane, it can take 20 minutes just to get off the plane. Then you walk (and walk and walk and walk) to get to passport control. I have never been able to get through passport control in less than an hour. Maybe that is just me and the times I usually arrive, but it always seems busy and slow. So with only 90 minutes, you now have 10 minutes to get to your next flight. And since boarding starts way before departure time, this might mean your seats have been given away even though you are there.

While 5+ hours might seem a very long time, it is much more preferable since it allows you to take your time and not worry about having to run to your next flight. By the time you get through everything on the way to your conneciton, you won't have all that much time to wory about. There are OK food options in the airport. Decent enough seating at most gates.

If I am connecting through Heathrow, I prefer to just spend the day I arrive at a nearby hotel and then fly out the next morning due to all the delays possible. I might even hop the tube into London for a nice dinner or show. I find this much more relaxing and enjoyable.
.

Thanks everyone, based on the feedback, I think we are going to go with another airline and have a different connecting airport. I appreciate your help!

Susan, I have only arrived and departed from Heathrow once, with no connections, so I have no direct experience to share. Other than to say I have NEVER walked so much in any airport in the world. There were directional signs with arrows, which said things like "30 more minutes to walk", "20 more minutes to walk", and so on. No need for a treadmill that day!

I agree with Larry -- hiking Heathrow is akin to one day on an RS tour with a "strenuous" activity level.

90 mins is ample for the vast majority of connections - some can easily be done under half an hour. Not sure though why you have been so secretive about what your flights/destinations are.

if both flights are with British Airways then it is very easy, other carriers not so sure

I second what Unclegus says. I have flown Philly to LHR and connected beyond via BA with pretty small time frames. This is all Terminal 5, however. Alternatively, I have given myself a day in London before moving on. If you have miles to spend, check out the Sofitel Heathrow 5. You can get off the plane, walk to the hotel inside the terminal, leave your bag, hop on your transport of choice, and arrive in central London fairly early. I recently made it to the Westminster Abbey opening time using the tube. All went smoothly. Alternatively, one could spend the day in Windsor or even try for a 2PM performance at the Globe.

I like the Heathrow Sofitel for convenience. There are food places near the entrance including a Marks and Spencer grocery store. And, it is very easy to wake up early the next morning and board your connecting flight. If you have to change terminals, even more reason to consider an overnight at the airport with a quick day trip to London or surrounds.

I have done the 5 hour layover at Heathrow and am not fond of that option. There are just so many magazine stores I can look at. I will do it on a flight home, but try not to on the way over.