Just had a look and £300 is very cheap for a new 11 speed groupset!
As for gears, I live in the lakes and use an 11/28 cassette (11 speed). I don’t often use the 28 though, I only bought this cassette as they had sold out of the 12/25. I do use a compact chainset though.
I would go for a 12/25 since it’s nearly a straight through. The bigger the range, the bigger the jumps between the gears (which can be annoying). I rarely use the 11t. I prefer to buy a cassette which is best for the majority of my riding, and just grind up the real steep stuff. Unless you’re riding up Hardknott all day, I’d go with a 25.