An Off Peak return is £61.40 with a Railcard. This is valid on any train between Ramsgate and London, includes a cross-London transfer and is valid on any train out of Euston.
On the return, it is valid on any train into Euston. It is valid on any train between London and Ramsgate.
Tickets from stations inside the Network Area to stations outside the Network Area are always valid on any train between the origin station & London, and on the return from London back to the origin station. Some ticket office staff and some guards do not seem to understand this. If challenged, ask them to look up the restriction and they will see it is valid.
As for which routes you can take, that is determined by the
Routeing Guide . Ramsgate is a Routeing Point. Blackpool North or South use Preston as a Routeing Point (as defined in the
Routeing Point Identifier page). The Permitted Routes are on maps LM+LR according to the
Permitted Route Identifier page. As this says "LONDON" that means all the routes from Ramsgate to London are permitted, plus all the routes from London to Preston are permitted.
Routes from Ramsgate to London are on maps:
HK
HS
LB+SD+LK
LD
LD+VC
LK
Routes from London to Preston are on maps:
BS+GC
CS
CS+NW
LM
MM+PS
The maps can be seen here:
http://www.atoc.org/rsp/_downloads/Routeing_Guide/Maps.asp