We missed our first port, Guernsey, as it was too rough for the pilot to get on, but it was warm on deck out of the wind (around 68 to 70 degrees). Cobh, Dublin and Liverpool were all about the same temperature which, for us Brits, was nice and definitely shorts weather. Belfast and Greenock were cool around the mid 60s (but still in shorts)! Orkney was the coldest - probably mid 50s and no shorts. Fleece jackets and hats were in evidence. Edinburgh was sunny, in the mid 60s but windy - we had lunch on the deck in shorts and t shirts. Le Havre was drizzle and misty first thing but warm and sunny (mid 60s) by lunchtime.
In summary, British weather is a lottery - be prepared!