Flying via Iceland Air, Seattle to Heathrow

They will check the bag all the way and give you the boarding pass for the next flight.
Unfortunately there is always the possibility that the flight from Seattle will be delayed 30 minutes 1 hour etc. enough to make you miss and end up on another later flight to london

I flew round trip between Minneapolis to London Gatwick on Icelandair last month and my connections at KEF were a little over an hour. I checked my bag through to Gatwick. I had plenty of time at KEF as I did not need to go through passport control, customs or security to make my connection. All I had to do was go to the gate (about a three minute walk). You will need to show both your passport and your boarding pass to the gate agent as you board your flight to LHR.

KEF is a compact airport, but it is a hub for Icelandair and WOW with lots of arrivals and departures in a very small window. You may find that the waiting areas at the gates are crowded and there are long lines for the bathrooms, shops and restaurants.

As far as passport control in the UK, make sure you obtain a landing card on the flight from KEF to LHR. The flight attendant will make an announcement prior to landing and go through the cabin to hand out cards. Fill it out and have it ready for the officials at LHR. It asks for your identifying information as well as the address where you will be staying in the UK. (I learned that many people no longer carry pen or pencil, and many don't have their destination address handy). On my flight most of the passengers didn't get the card (slept through the announcement or weren't paying attention), and they were not allowed to get in line at passport control until they had completed the card.

Enjoy your trip.

Although I have never been through KEF airport, as I understand it, you shouldn't have to clear passport control or customs when connecting US to LHR. Normally if flying onward to Paris, Rome, etc.. you would clear passport control in KEF as your entry into the Schengen zone and then a simple walk through exit at customs at your final destination.

But since the UK isnt part of Schengen, you only clear passport control when entering UK borders at LHR. Come to think of it, you probably won't even have a security check in KEF as it is such a small airport and connecting non-Schengen to non-Schengen flights. 55 min should be more than plenty of time provided your first flight arrives on time.

Have fun

Icelandair are experts at the fast turnaround. Nothing to worry about.

Since so many people connect at KEF for other flights, they might even hold the flight a short while to make sure most everyone makes it.

One thing, go to your assigned gate and check it out before going shopping or to get something. KEF is overcrowded and in some cases you will have to be bussed out to your plane.

I was there three months ago and there was quite a bit of construction at the terminal so be aware of that.

No problem whatsoever. I just did that route. Felt like we had plenty of time to make our connection. Its so busy at that airport you'll be glad you have a short layover.

As Frank said, Icelandair has plenty of practice making these connections since all their flights trans-Atlantic go through that airport.

I flew Seattle to Heathrow last year. Very tight connection in KEF. It was a breeze. The airport is small, security is easily dealt with (you basically walk through a doorway with security watching you) and onto the next plane. As my plane was a tad late, they actually held the other flight. If there was an airport where I was comfortable with a 55 minute turn around, KEF would be it.

I've flown Iceland Air out of Seattle twice and haven't had an issue with the short connection, even with only 45 minutes between flights. One time we arrived a bit late and it was still fine. I checked luggage both times, but I suppose it's safer to carry it on to ensure it does not get lost. Have a great trip!

Thanks everyone!