Some relatives of mine are needing to travel from Heathrow to Gatwick -
obviously there's the coach option (that presumably goes around the M25) or
the tube/train option via Victoria.

We've basically worked out that it looks like train is likely to be the
cheapest option (they have a Family Railcard and will be travelling
off-peak), and the fare (2 adults, 1 child) looks to be just over £20 for
the Victoria-Gatwick leg, plus tube fares.

Anyway, my question is - could they get a through ticket that includes
travelcards, just as they could do if they were starting outside London and
heading in? (i.e. their journey in reverse)

Cheers,

Peter
--
"For want of the price of tea and a slice, the old man died."