I did the Seattle to Sprucegoose in a day and it is a long drive but totally doo-able. Or you could just keep the night free and check in to a Motel on the way back to SEA. But you must visit the Sprucegoose Museum. It is excellent.
Make sure you book the Boeing Tour before you leave the UK if you know what day you can go just in case it is busy. A tip is to book the very last tour as most tour buses don;t bring folk in that late in the day. When I first went there was only 4 of us on the tour as opposed to the 30+ they usually have. Made the tour much more interesting as the guide would mention a lot of things they wouldn't normally talk about on the main tour.
And make sure you get to the Museum of Flight. The old Boeing Wooden building was closed when I was there last but it is still a great place to visit.
Check out the Float Planes from Lake Union to Victoria in BC. Great 1hr flight and a great day out.
Will post more later as in a rush to head out.
Rich