wrote:
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)

Probably wouldn't be any cheaper, certainly if you all have oyster
cards.

If you do take the tube, Piccadilly to Barons Court, District to
Victoria would be the best move (cross platform at Barons Court).
However it will cost £5-10 for the tube trip.

On the return journey (assuming it's off-peak) a one day travel card is
available FROM Gatwick to zones 1-6. Cost £10-£11 depending on which
TOC is used to London.

Good idea - just checked and it's £16.60 for the three of them, restricted
to FCC services, or £25.40 going via Victoria. What's Blackfriars like for
getting from the tube up to the mainline station - any escalators etc? It's
not a return journey btw, they're just heading to Gatwick from Heathrow.

Also, will it present a problem if they were to get return travelcards
starting from Gatwick and the first time they'd put it through a barrier was
at Heathrow? I'm guessing probably not, as they could well have showed them
to a regular ticket inspector on the barriers at Blackfriars/London Bridge
and only used them on buses, hence not going through any machines anyway.
Any comments on this?

However, outward no such beast. A possible solution (assuming travel is
late enough in the day to be on the return leg) is to buy the Gatwick
travel card in advance. Otherwise a zone 1-6 travelcard and a boundary
zone 6 to Gatwick extension, or the Feltham bus - Clapham Junction
option.

Well, the £16.60 option from Gatwick looks the cheapest option, though it
restricts them to First Capital Connect services, which take around 45
minutes from Blackfriars, plus the extra time on the tube, though the price
is good and they're in no major rush. Providing Blackfriars has some
escalators up to the mainline station.

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