We sailed the Celebrity Century last May to the Norwegian Fijords. We flew to Amsterdam and boarded the ship there.
You could also take a look at the Hurtigruten cruises which visit a lot of places that the larger ships cannot get to. You have to fly out and back for these as they do not leave from the UK. They are a bit more expensive and do not have quite the same facilities as companies like Celebrity and do not have much if anything at all in the way of entertainment. Pop into your local travel agency and ask them for a brochure, they could also tell you what other companies you could look at. We went with P&O last year and it was a lovely trip, the scenery is fantastic. Good luck.