I cruised to Norway in Agust and it rained for 6 days out of 7, and wasn't especially mild either. I'd pack t-shirts, a warm jumper and everything in between.
The writer from Birmingham is so right about Iceland as the weather there can so often be rainy but Norway is not as bad and you can have it, as here in Britain, really hot and cloudless in July. The main trouble is that most of the weather in these regions comes from the Atlantic and in summer that is so unpredictable.... Nei.
We did Iceland and Norway in July a couple of years ago and it was warm and sunny for the trip to Iceland (19 degs in Reykjavik) and then cooler and more cloudy as we crossed to Norway....a bit drizzly once there. We packed layers... but nothing more than t shirts and cropped trousers with a micro fleece and cagoule and shorts and vest top for when it was warmer. We loved it and did a trip from Reykjavik which took in a coastal area with high cliffs and birds and also the Blue Lagoon for a brief stop...but we were on Sea Princess not Thomson..so perhaps someone else can help with the trip bit of your question!