Flying into Seattle from Europe. 2 hour connection time enough?

Yes, Kirsten, you should be fine although it would certainly help if you download "Mobile Passport" which will get you through Passport Control in about 5 minutes. It would also help if you only have carry-on because the greatest delay will be waiting for your bag at the baggage carousel. (That would be the greatest delay if you have Mobile Passport on your phone.)
My last trip I actually stayed in Seattle but with the Mobile Passport and waiting for my cargo bag, I was out the door in half an hour after landing! I was shocked because that's never happened before!
EDIT: http://mobilepassport.us/

We will have only carry-on.
I had no idea about mobile passport. That's amazing! Thanks a bunch!

You're welcome!

Your London flight will come in to the S terminal, which is the international one. You will need to tKe the underground train to the main terminal to reach the gate for your next flight. Which airline? We can probably tell you which concourse your flight will use.

I went through about that time of day a couple of weeks ago and it was quick, except for a too-long wait at baggage claim, which won't be a problem for you. You should be OK assuming that your flight's on time. If your connecting flight is all the way up to the north satellite (an "N" gate), you'll have to take two underground trains.

I didn't know about Mobile Passport, looks pretty good if you can get wifi at the airport. Makes me wonder why I paid $100 for Global Entry last year.

One point you left out: are your flights all booked on one ticket? If so, then you're fine. If not...then if your flight to SEA is late, you do run the risk of missing your connection, in which case you would be out of luck. If it's all booked on a single ticket, then the airline would need to accommodate you for the onward connection even if you miss your originally scheduled flight.