I have yet to find a way for non-rail-staff to access P2 Viewer, but I'd be interested to hear if there is a legitimately-available tool allowing folks to do so.
Rail-related open data feeds have been used to create some pretty awesome sites such as OpenTrainTimes, which features maps of certain areas. These show live positions of trains, somewhat similarly to P2, but without freight/NR trains' headcodes or all of the logs that P2 can display.
http://opentraintimes.com/maps
I note you used to work at Clapham Junction, and so there may be some maps there to interest you.
In addition, though the Briefcase Rail Train View map versions do not fully cover that particular area (some are in development, but I won't link to those in case they become confusing), much of the rest of the Brighton Mainline and surrounding routes can be viewed live through them:
http://www.sussextrains.co.uk/ (roughly South of Gatwick Airport / Eridge) and
http://www.sussextrains.co.uk/sandbox (North of Gatwick Airport, plus some branches - not including lines North of Norwood Junction or from around the Streatham Common area Northwards).
(Note also the "added extra" commentaries on various aspects of the BML!
)