I believe London FIR have a radar feed available to them on the wings these days, but they are not a radar service, so no identification nor separation services are available. Why have it then? I cant remember, but I guess something to do with aiding visualising for FIR charges to get clearances and coordinations in good time. Maybe a hint to them that an FIR squawk is nearing a controlled airspace boundary?
No doubt a real FISO will be along shortly to give the full story.