On May 25, 5:33 pm, wrote:
On 25 May, 16:54, JL wrote:

On an Any Permitted ticket, you can go via London provided that you get
to Reading via Staines (e.g. Gatwick to London Bridge, W&C line,
Waterloo to Reading).

I'm sure a Not London ticket would also let you go Gatwick, Clapham
Junction (change), Reading (via Staines). But if it's an Any Permitted
ticket, I'm confident there's nothing stopping you going via
Paddington...

The only thing stopping you going via Paddington is a omission (?) in
the Routing Guide which stops Trainline-derived websites issuing the
ticket. Basically, Reading to Gatwick is only valid on maps CS (via
Guildford or Kensington Olympia) or LB+WX (via Staines and Clapham
Junction). It should be mapped LONDON too but hasn't been in recent
versions of the Routing Guide. It would be interesting to know
whether the barriers at Paddington knew about this.

There are two fares from Reading to Gatwick (plus the Cross Country
only AP fares), "Gomshall" and "Any Permitted +". The first is only
valid via Guildford, the second allows travel on any permitted route.
A specific "Not London" or "London" flow doesn't exist.

Reading to Three Bridges is routed "Not London", "Not Gatwick Exp +"
and "Any Permitted +". Three Bridges is a routing point and the
routing guide allows travel on routes "LONDON", "CS" and "LB+WX". The
Not London fare in this case is valid via each of Guildford, Staines
and Kensington Olympia and would make a better purchase for the OP.

Jonathan

Appreciate this response (and everyone else who has contributed),
thank you