A lot of the above replies appear to have ignored the restriction codes of the various advanced tickets.
For the Exeter - Barking query
The restriction code code for all the +AP LONDN HONITON tickets is SW
Restriction : SW
Valid on South West Trains (TOC Code
WT) and connecting services only.
The restriction codes for the +AP LONDN READING are IF, IS, ZJ, ZK,
Which all read:
Valid on First Great Western (TOC Code:GWA/GRW) and most connecting services (see below)
NOT valid on SouthEastern High Speed services (HS1) arriving at London St Pancras International before 0959. or departing London St Pancras
International between 1600 and 1859, Monday to Friday.
Therefore your ticket is valid on connecting services as is not restricted to booked trains only.
Regarding the original query the routes available for advanced tickets (Truro - London Terminals) have different restrictions.
If the ticket is routed GREAT WESTN ONLY the restriction codes are QF, QS, ZH, ZI
QF & QS read:
Valid on First Great Western (TOC Code: GWR) services only.
ZH reads
Valid on First Great Western (TOC Code:GWA/GRW) and most connecting services (see below)
NOT valid on SouthEastern High Speed services (HS1) arriving at London St Pancras International before 0959. or departing London St Pancras
International between 1600 and 1859, Monday to Friday.
ZI reads
Date and time shown on accompanying reservation.
If the ticket is routes AP SLOUGH the restriction codes are IF, IS, ZJ, ZK,
Which all read:
Valid on First Great Western (TOC Code:GWA/GRW) and most connecting services (see below)
NOT valid on SouthEastern High Speed services (HS1) arriving at London St Pancras International before 0959. or departing London St Pancras
International between 1600 and 1859, Monday to Friday.
So what is allowed depends on what price ticket and what route ticket was purchased.
Peter