We camped up in Glasgow for 3 weeks a few years back in August and I took extra duvets and fleece blankets for everyone. I put them in those space saver bags that you suck the air out of so they didn't take up much room (we have one of those pumps you plug into the car lighter). The kids were perfectly fine with their sleeping bags and I had their extra duvets
This year in July we 'glamped' in a prospectors tent on the borders with France and Belgium. The tent had a wood burning stove which we lit in the evening and then just let it burn down over night and we were plenty warm enough.