My understanding (which may be incorrect) has been that London Info have a radar screen which shows only traffic with their 7xxx squawks.
This helps them stop CAS busts.
It ought to also enable them to offer some assistance in an emergency situation - better than just listening without having a clue where somebody is.
I could see why this was done: the people are not radar qualified ATCOs, or possibly not ATCOs at all, so they cannot be given the job of an officially radar qualified person. Those are the rules...
But in the context of this thread which is a flight under own navigation, I cannot see a problem doing the whole lot with the radio tuned to London Information.
If I was doing it myself, I would do it with a listening watch only, and calling up known
radar services along the way.
The "problem" with actually talking to London Info used to be that they would keep asking for estimates to such and such a waypoint, which was time consuming for a pilot who was navigating by radio (GPS/VOR etc, where you are getting continuous lateral guidance and thus have no need for managing ETAs to waypoints). With a decent GPS, this is not an issue because you see, next to each waypoint, the ETA, and you can just instantly read it off.
I don't know whether London Info still do this; it would seem quite funny if they had to "cover up" their traffic visibility by asking traffic where it is
My recent experience of speaking to London Info is that they don't do that anymore, but my experience is limited to the brief times during flying back to the UK from say France, where one got a handover to London Info instead of London Control, but since one is descending anyway this doesn't matter. To me, the lack of queries together with a well timed instruction to descend below FL075, etc, indicated they could see exactly where I was, but of course nothing was ever spoken on the radio to suggest that.