I don't think you'll find many cruises going from Harwich. I've been on a few cruises and find the dressing up posh thing a bit intimidating. Also the tips, it can get a bit ridiculous.
The best 'cruise' I went on was with Hurtigruten who sail along the Norwegian fjords. Very informal, no entertainment, just read your book and enjoy the scenery, lots of stops if you want to get off for a while, but most stops are very short.
You'd have to get a short flight to Bergen to get there.