We can actually get relatively cheap train fares compared to most north Londoners by purchasing a train only ticket without access to the tube. This works well for people working in London Bridge, Blackfriars, Canon Street, Waterloo, possibly even Kings Cross, and areas close to main line stations. For commuters to Canary Wharf.you can by a travelcard avoiding zone 1. These are two ways to pay less for you travel without the need for rezoning.
I realise that parking very close to HOP station can be difficult, and i do have sympathy. But if HOP was in zone 2 I would personally be very tempted to drive from Forest Hill to park there. Financially it would be crazy for me not to do that and would avoid walking up a hill every evening. The only question then is whether i would walk 150m or 300m after parking my car close to the station - it is just about how much i want to save money. The only thing that stops me doing this in East Dulwich is they have worse train services than Forest Hill.
Moving HOP to zone 2 makes a lot of sense for people in Forest Hill. Less commuters parking in our roads and we can drive to HOP where there are loads of residential streets to park close to the station, allowing for cheaper travel to London.
Imposing a CPZ would need to be quite large in such a situation and people in HOP would need to pay more to prevent me enjoying the same cheap travel that they would from the most remote zone 2 station other than Stratford.