The a/c was intercepted before it got to London. It was just making landfall.
There was a flight plan in the system, so the flight wasn't a surprise. If the previous controllers had lost comms with the a/c, obviously we are reliant on them informing us before it gets to our airspace. Occasionally, adjacent units will let the situation progress further than perhaps we would before saying anything. The a/c could well have reached the normal transfer of communications point without the sector being pre-noted there was a potential radio fail, or loss of comms. This obviously delays the QRA launch by a few minutes.
However, the pilot was intercepted well before overflying significant urban areas, and showed compliance in following orders.