I guess Northolt has been a well kept secret because no instructor I have ever met has even remotely suggested it.
On the "London LARS" thingy, what they should do IMHO is to make the point that once you are issued the squawk, you won't be bothered except with traffic reports.
Whereas London Info (who I use for a listening watch only) will ask you for position reports and that is unnecessary work if you are comfortably sitting there tracking the GPS track.