Someone I know who has access to the FRPP website confirmed to me
that:

For domestic tickets, a journey from the Southeastern area to stations
beyond London which would normally be routed via Ashford will allow
travel by "Any Permitted Route", and that would be valid for travel on
the Javelin preview service without a supplement. A local journey
Ashford - London would need the supplement, however.

Now here are the prices for tickets from Ashford to London St. Pancras
with the surcharge.

Anytime Single (SDS) £26.60
Anytime Day Return (SDR) £48.70
(HS Upgrade Anytime Single (HUA) £4.40)
(HS Upgrade Anytime Return (HUB) £8.10)

However, if you get a ticket from Ashford to Kentish Town that is any
route permitted, and therefore allows you to travel on the fast
Southeastern services, the prices are as follows:

ASHFORD INTL 5004 - KENTISH TOWN 1553
(Derived from ASHFORD INTL 5004 - W HAMPSTEAD STNS 0265)
rte Any Permitted

SET
FDS S 33.90
FDR R 67.80
SDS S 22.60
SDR R 42.50
SVR 4A R 28.60
CDR C4 R 23.10

Looks like you can save yourself £4 on a single,, or £6.20 on an
anytime return... or am I missing something?