You can see my posts on choosing things for trip to Ireland. Temps there were 40's-60, but still layers are the thing. You will probably be taking some kind of raincoat/rain jacket--for me, a hip-length sporty rain jacket was fine, since we were very casual, but if you need to dress more smartly, a trench would work. The good thing about a nice rain coat/jacket is it serves as a good insulating layer on dry days as well as on rainy.
I used "long underwear" in silk and merino(2 types) and footless tights as layers under pants, since I run really cold. My pants and tops choices,.by themselves without extra layers, would have worked for me even into 70's, but warmer-natured folks would want short-sleeves and maybe shorts or skirts. Also, then the outfit itself "speaks" more, whereas actually I stayed rather bundled up during my trip.
Worst case scenario is, seems to me, is that if it's cooler, you end up wearing all your heavier items or layers all the time, or warmer, vice versa.
I took things I could hand wash and dry pretty easily, and did a little of that, mostly underlayers, and also we were not gone so long, and I had overpacked a bit so had a little more of everything than I needed for a true minimalist packer. Many of the blogs I read said just to plan on doing some laundry at laundromat, or via hotel facilities/services. So, interestingly that might influence what I pack for a trip, such as sturdier items I "trust" in commercial laundering vs things that pack well but I would only hand wash and dry.
