Unfortunately, UK LARS used to be a joined up system but now there are some large gaps in radar cover. Without radar cover, all that can be expected is a basic service, often from a distant ground unit, such as London or Scottish Information. Hence my earlier point that it makes little difference whether a written flight plan was filed or not. A basic service can however be used to good advantage for SAR purposes, should the worst happen (keeping up to date with accurate position reports is what I mean).
I would rather remain in "live" R/T contact than rely on a written flight plan which is at best a summary of my proposed route, on a piece of paper in someone's in tray and which may or may not be of use afterwards.