We are flying AA, arriving at LHR at terminal 3. Our connecting flight to Rome on BA leaves from terminal 1. What is the terminal transfer like and how much time will it take?

Give yourself a lot of time. LHR is HUGE and you walk a LONG way. I was just through LHR last month..had to go from terminal 4 to 1. Got a map of LHR and the suggested time between terminals was 75 minutes, and that included a 15 minute bus ride. It did NOT include going through security etc. Just give yourself enough time.

You do need to take a bus from Terminal to Terminal which can be up to a 15-minute bus wait and you do need to go through security again. Depending on how busy the airport is, I did it back on January 2 in 45-minutes, but it could take close to two hours if the airport is real busy.

If you buy anything at the airport, try to use a credit card since you probably don't want to deal with British Pounds and if they do take U.S. Dollars, the change will be given in pounds.

Why do people never look at the airport website?

www.heathrowairport.com

Click on "airport information", then "flying into Heathrow", then "connecting passengers"

BAA's LHR website is, IMO, part of the problem, Alan.

If you can't use the FCC (for example a landside transfer from T1 to T4 - with bags) the website doesn't tell you about the rats' maze of tunnels, bag carts that can't be taken on the HEX, poorly-marked lifts...

But just watch jet-lagged families with cranky kids and mounds of bags try to negotiate the transfer yoga. It is to weep. They sure can't pull up the "interactive maps" that don't show the tunnels when they're standing there looking bewildered.

Airport websites are only helpful, up to date, and accurate by accident, now and then. They are most useful if you've already BEEN to an airport and need to refresh your memory about something.

Thank you all for your replies; I have a much better picture of what lies ahead.
And as to why "people never look at the airline website?", well, overlooking the unappreciated sarcasm, and having looked at the website, my purpose was to get information from people who have actually walked the walked.
So again, thank you.

After deplaning at T3, follow the signs that say Flight Connections (as opposed to Immigration). You'll head downstairs and take a shuttle to T1. After getting off the shuttle, you'll go through a security check. Right after security check is the BA transfer desk if you need to pick up your BA boarding pass. I've done the transfer in 30 mins or so during the afternoon when it wasn't busy and there was no line at security.

We had to do a LHR terminal transfer around the 7th of January. We landed in Terminal 2 I think and had to go to terminal 3. Our nightmare began with our Iberia flight arriving 45 min late in London. We then had to make the mad rush through security, then make it to the Bus to change terminals. The line was backed up all the way up the stairs. There were no busses there to pick up passengers for the transfer. The lady that was stationed there was calling and saying that they weren't responding to her request for more busses. If you have a tight connection, make sure you let them know at security and at the bus pick up. They will probably check your boarding pass, but it will get you to the head of the line. You have to do a lot of walking, so make sure you have at least 2 hours between flights and expect delays in getting to your next terminal. LHR is a disorganized nightmare in my opinion if you have to make a terminal transfer. Good luck.