AA: SEA-INV 2.63cpm
 

SEA-INV is 355 USD lowest but there's a SEA-DFW-LHR-INV routing which is 13820 miles and 364 USD -- that's the best I could get out of it.

This is on Expedia.

Departs SEA: April 2, 23, 25; May 5, 7
Returns: April 9, 10, 11, 15, 23, 24, 25, 26, 28, 30; May 1-5, 7-9, 12, 14-15, 20, 23

This is like Dublin, for some reason INV seems to be cheap pretty often, anyone know why?

FWIW, I'm seeing multiple dates in your range (but not specifically your dates) for around $390USD, both on AA/BA metal and DL/KL.

Caveat, and forgive my ignorance if Expedia is wrong and I don't know any better...seems like the BA (O) fares don't include a checked bag? I could handle Scotland for a week carry on, but I know lots of people would wanna check a bag for something like this.

The price more than doubles when trying ex-YVR btw... *face palm*

fares to/from INV aren't subject to APD which makes a difference mainly for premium cabins.