When backpacking or mountaineering in the bridge seasons, which sleeping bag and mat to carry is a tricky call. If it's warm then less is more, but of course there are no guarantees in the UK. For three season use look for a combo that can take you down towards freezing level in reasonable comfort, because you just never know. As someone who sleeps cold, and has a habit of camping on windy Scottish summits, this goes double for me.