This is actually quite straightforward
NRE says:
Outward: ROUTE OF TICKET LONDON NOT UND - You must travel via London. Transfer between London Stations is available on the Docklands Light Railway and First Capital Connect (Thameslink Route) or by foot. Travel is not available on London Underground. This fare is only valid on trains travelling between certain times.
Ramsgate - Clapham Junction (returning on a CLJ - RAM return) has the route LONDON
"If the routeing code is "LONDON", for all journeys via London you will need to
cross reference routeing codes applicable "to London" with the code for the
"from London" leg of the journey.
If the routeing code is "LONDON", journeys include the cost of cross-London
transfer either by London Underground or Thameslink services. In all cases
the transfer points should be along the correct line of route given by the
‘permitted route’ map combinations. "
So the permitted routes are:
Clapham Junction - London Terminals (any permitted route)
London Terminals - Ramsgate (any permitted route)
You may join these two permitted routes at any point within London along the permitted routes
"on the Docklands Light Railway and First Capital Connect (Thameslink Route) or by foot"
Ramsgate - London has numerous permitted routes.
HK HS LD LD+VC LK LK+SD+LB
No surprise to learn that HS covers the high-speed train
http://www.atoc.org/clientfiles/File/Maps.pdf#page=48
So your ticket is valid from Ramsgate to St Pancras.
To get from St P to Clapham Junction you could travel from St Pancras to London Bridge by FCC, from London Bridge to East Croydon, and then back to CLJ.
Or you could simply buy an underground ticket from St Pancras to Vauxhall (aka 'on foot') and continue from there to CLJ by train.
It's valid in both directions to start/finish short from St Pancras, and is cheaper than +HS ticket from London Terminals.
It's also cheaper to buy a CDR CLJ-RAM (£31.90) with Return Travel 'By Any Train' and throw away the outbound, than to buy a RAM-London Terminals SDS (£38.10) or CDS (£36.20).