Our room in Rome is just northeast of the Vatican entrance, near the Risorgimento/S. Pietro? tram station. We have an 800 flight (British Airways) from the Rome airport (FUM) to LHR, and a 1300 flight from LHR to Seattle. (Different booking). We have only carry-on luggage. Two questions: What is the best means to get to the airport in the morning, and is the 3-hour time at LHR enough to change planes on different bookings?
You will need to be at the airport around 6 and that is just a hair early for the trains. I would have the hotel to arrange either a shared shuttle or a taxi at 5:30.
Three hours in LHR is cutting it a little close since you are now doing check in for an international flight and they recommend you arrive at LHR to start the process three hours in advance. If everything goes well you will be OK, if not... One suggestion, IF the flight from LHR to SEA is also on BA see if the BA agent in Rome will check you all the way through. That way you won't have to go to the check in counter in London. I am not sure they will, I have had Delta do this on domestic flights, and Delta has done it for my mother on international flights (she booked a tour out of Atlanta, I gave her a frequent flyer ticket to Atlanta... but just in case I also gave her a LONG connection both ways in Atlanta)
Sadly, no, the LHR->SEA flight is from IcelandAir.