When arriving from SEA to LHR if we have nothing to declare do we have to fill our a customs form? Can these be printed on line so they could be filled out ahead of time?
I'm not 100% certain about the UK... but for most of the Schengen Zone, you just walk through customs unless you have something specifically to . And note that customs is a separate station from immigration.
It has been a while since I went to the UK, but several years ago, there was an immigration form to fill out that included a customs declaration. I do not believe that it was available on line. The form asks basic information as well as the address of where you will be staying. If you are just in transit through LHR, then there is no form required.
Mary - you can find everything you need to know here: http://www.ukba.homeoffice.gov.uk/travellingtotheuk/Enteringtheuk/
Every time I have flown to LHR, they just give you a customs/immigration form on the plane. There's plenty of time to fill it out before landing.