If you hold a UK PPL with no instrument qualification, it is quite legal to fly IFR outside controlled airspace provided you do not exceed the Schedule 8 Legal Minima of 3K vis, and if below 150 Kts, clear of cloud and insight of the surface.
Simpler than that, I think:
If you hold a UK PPL with no instrument qualification, it is quite legal to fly IFR outside controlled airspace provided you do not fly in a visibility less than 3 km.