Just to confirm -- I was able to book MAN-LHR-YVR-SEA-LHR-NCL as jamestg suggested -- thanks. I also tried a return from NCL but that was more expensive.i
Leeds is a bit closer to MAN than NCL and no need to switch to metro (except on a Sunday at present when one has to walk/metro between Victoria and Picadilly owing to engineering works -- but that's another story). I'll be interested to try NCL.