Helicopters operating in the LHR and or City zones, in order to operate to and from London Heliport (Battersea), a VFR only location are required to be flown under SVFR / VFR as appropriate. This involves the principle of see and avoid. If you can't see, it's possible you won't avoid, irrespective of how good your navigation kit is.
Isn't that rather the point?
If the only things that can penetrate the gloom are high intensity beacons, then you cannot 'see' by any practical definition of the word, and you shouldn't be up there.