I have done the British Isles cruise 2x in August. We tried watching the outdoors movie on the sea day, we were only able to sit for 10 minutes with jackets and a blanket as it was cold and windy.
In May 2022, I sailed out Southampton for a Norwegian Fjords cruise. On the sea days, it was EXTREMELY COLD and WINDY, no one was sitting out. Passing the open outdoor automatic door at the buffet, you could feel the COLD air. I am from Canada, it felt worst than our winter weather.
I would not spend money for the sanctuary chairs. If you want a sanctuary chair, go on a sea day, and see if any chairs available. British Isles cruise weather is not like a Caribbean cruise especially May weather.