I've never transited through Heathrow en route to another destination, as I will be doing later this year on the way to Rome. My flight from IAD has been changed so that now I have only a 2 hour layover at Heathrow. Will I go through immigration at Heathrow? (Last time, it took forever....) Anything else? Will 2 hours be enough time to connect?

Unless you're traveling on separate tickets (are you?) you won't go through UK immigration. You'll go through a typical security check, then on to your departing gate. What airline(s) are you using? Two hours is generally adequate, even with a terminal change.

Yes - you need to tell us more -- which airline(s)? One booking or two?

You can get step by step instructions for your particular transfer/connection along with minimum time required on the Heathrow Airport website: https://www.heathrow.com/connecting-flights

Just enter your date of travel, airline/flight info and you'll get all the info you'll need.

I'm flying BA on a single booking. And I'll check that website RoamsAround cited. Thanks!

Most - but not all - BA flights use T-5. If you land anywhere near on time you should be fine.

BA use T5 for both IAD arrivals and FCO departures, so just go through security and enjoy(?) the T5 shopping mall or lounges if you're riding in the pointy end.

Follow the signs for Flight Connections and be aware that some flights from T5 need a transit train to get to the correct gate.

https://www.britishairways.com/en-gb...ht-connections