Having a look to see if there are any deals for this year.

first up and a recent price drop is Emerald Princess

12nights from £829pp 3&4 from £500pp

Inside Passage (Roundtrip Los Angeles with Glacier Bay)
Roundtrip from Los Angeles, California
Emerald Princess Fri, Apr 27, 2018

Cruiseline air is £530 return (direct LHR<->LAX on AA/BA flights )
Virgin £490 Norwegian(LGW) £380(bags extra)

flexible flight dates for pre/post cruise options.