Read this: Signal failure - This is Somerset

Summary: Freeview customers saw their programmes replaced by French channels as the weather played havoc with TV signals. Broadcasters blamed a high pressure weather system which can cause signals from foreign channels to 'bounce' off their normal course, a phenomenon called 'continental interference'. Satellite services, such as Sky, are not affected by the phenomenon as their signals are beamed from space. - thisissomerset.co.uk