Colin Rosenstiel wrote

Does anyone here understand the logic by which TOCs decide the addon
cost
of Day Travelcards over Day Returns to London Terminals?

I don't know.

Looking at a random selection of towns and cities around London most
seem
to be in the range £2.50-£4.50 (before railcard discounts). All
except
FCC, that is. The Huntingdon addon is £5 while Cambridge seems to
have to
highest of all at £6.

Above £4.50 and the point of Travelcards is destroyed for most users
who
only make 2 tube journeys and have railcards and Oyster.

But I don't follow your logic, why £4.50 ?

(2+z1 Oyster tube, £3.00 - Oyster cap Z1-2 £4.80, £3.10 with railcard)

(Walton-on-Thames off-peak £11.60-£7.60 super-off-peak £10.00-£6.50 so
£4.00 and £3.50).

--
Mike D