Army issue sleeping bag, bivvi bag and rollmat. Sleeping bag in bivvi bag, bivvi bag on top of rollmat. In 15 years and counting, I’ve never had a problem with that setup on countless exercises and deployments around the world in a wide variety of terrains and climates. I imagine you could get all three items off eBay for under £50.
Admittedly the issue sleeping bag is a little on the bulky side, even when packed into the compression sack. If you’re taking your sleeping gear in a car to then camp at a static location (often do this at Dolgellau when visiting Coed y Brenin for a weekend), then this isn’t really an issue. If you’re carrying stuff on a bike though, most of Snugpak’s Softie range of sleeping bags compress down pretty small.