Any permitted route question

First off, there are many flows where there is no Anytime Return, particularly in the former Network SouthEast area. Such journeys, out one day and back another, both at peak times, have to be done with 2x Anytime Day Singles. There are also some flows in other parts of the country where the Off Peak Return has no time restrictions, meaning there is no need for an Anytime Return.

Now as to the routeing question.

Datchet is not a Routeing Point in it's own right, but is associated with Slough and Staines.

First thing to check is the shortest route between origin and destination as this is always valid. That is:
Datchet, Sunnymeads, Wraysbury, Staines, Ashford (Surrey), Feltham, Feltham Junction, Whitton Junction, Whitton, Twickenham, St Margarets (Greater London), Richmond, North Sheen, Mortlake, Barnes, Putney, Wandsworth Town, Clapham Junction, Wandsworth Common, Balham, Streatham Common, Norbury, Thornton Heath, Selhurst, East Croydon, South Croydon, Purley Oaks, Purley, Earlswood (Surrey), Salfords, Horley, Gatwick Airport
So essentially, Datchet to Clapham Junction, change, then Clapham Junction to Gatwick Airport.

Next you need to do a fare's check on the associated Routeing Points. If the fare from the Routeing Point is more expensive than the fare from the origin station then it is not permitted to use that Routeing Point. That isn't straightforward as it depends on the ticket type being purchased. Some fares between Datchet - Gatwick are cheaper than Slough - Datchet meaning Slough wouldn't be an appropriate Routeing Point, but others would pass the fare's check rule, allowing Slough as an appropriate Routeing Point. We'll leave that thorny issue to one side though as you want to travel via Weybridge which logically means using the Staines Routeing Point.

So, is Staines Routeing Point to Gatwick Airport (which is also a Routeing Point) via Weybridge a permitted route?

First we look at the Routeing Guide's Permitted Route Identifier. From that we see that Staines-Gatwick Airport has two Permitted Routes. The first is LONDON and the other is to be traced on Routeing Guide maps WX+LB.

LONDON means you take any permitted route between origin Routeing Point and London, then any permitted route from London to destination Routeing Point. We can discount that for your query as there are no routes from Staines to LONDON via Weybridge. The only permitted route is that which can be traced on Map WX.

So, that leaves us with the map combination WX+LB. As Map WX is the same one as used for Staines-LONDON then sadly that also rules out travelling via Weybridge.

In summary. Datchet to Gatwick Airport via Weybridge is not a permitted route.

If you wish to travel via Weybridge then you will need two tickets.