And a few narrow gauge lines missing from that list in both Surrey and Kent.
Romney, Hythe & Dymchiuch
Siitingbourne & Kemsley
Bredgar & Wormshill
Old Kiln Rlwy
Hampton & Kempton Waterworks
As much as many of us would like to see Narrow gauge included on the map as it was for a long time included on railway maps, i think from what i can interpret is that the railway must have either a physical connection to the mainline, or in the case of the East Kent and the Spa valley at least within the National rail station grounds. Many of these lines will not count as SKLR station for Sittingbourne (Sittingbourne viaduct) is halfway across town from the main station, and Bredgar is pretty much a guys collection with open days.
However, with regards to the RHDR I will say that if it still had a station in the Dungeness area, it would count as it was present on the Southern railway maps from 1927 till just after the war.
One last piece of note is this map here.
https://shop.thisismikehall.com/products/great-britain-railway-map
Though this some guys artwork that you can buy the brilliant thing is that he tries and includes some heritage railways onto the map, however I don't expect it to be on National rail anytime soon and more likely in a heritage railway giftshop.