Ironically, despite having a pair of perfectly good drysuits in the dive shed, I'm currently eyeing up a 5mm wetsuit for UK snorkelling purposes. I already have a 3mm shortie which is OK for shorter sessions on sunny days, but I'm getting cold after an hour or so and that's in the height of summer.
I started my diving life in a 7+7mm semidry suit, which by today's standards would be considered a wetsuit! I did about 2 years, diving all year round and in the summer it was bearable, almost pleasant at times. In the winter, 20 minutes was max endurance and my air consumption was hilarious. It was unpleasant, but I was 17, skint and had a burning passion to get underwater, no matter how painful it was.
These days, a tri-lam drysuit is my weapon of choice for UK diving, no matter how warm it gets. The wetsuit is the better option for snorkelling and mucking about in the sea, but as I say, I'm now considering a full length one in 5mm. I can add the shortie over the top if I need to for the colder seasons.