I think these maps are saying that if I fly from Cambridge to overhead Felixstowe then even on a weekday there's no LARS coverage at all for pretty well the entire trip.
Well, if you will pick one of the few bits of England without LARS...
However, once out of range of Cambridge App, try Lakenheath, then Wattisham. Neither is formally LARS, but both normally give you some sort of service. Wattisham strictly Mon-Fri 9-5, but they tend to know about their helicopters that go scurrying around the county at low level.
PS - I know the LARS map doesn't give much detail of coverage, but I go on the basis that if I call they wrong one, they'll soon put me right.