If I buy a ticket to “LONDON TERMINALS”, such as this ticket, is that ticket valid to any London terminal, or only until the first one I arrive at? For example, from Earley such a ticket is either via Staines to London Victoria, or via Reading to London Paddington. Is this ticket then valid for subsequent travel to other London terminals such as London Euston, London St. Pancras, or London Liverpool Street?
London Terminals is better defined as the first National Rail terminal you travel to rather then allow travel between terminii. The exceptions are the old Southern stations which are interconnected and the Moorgate line and definitly not London Underground.
The definition is on the National Rail website.
Tickets issued for travel to/from London usually show 'London Terminals' as the destination/origin rather than naming a specific station. This is because the ticket is valid to more than one London Terminal station provided it’s on any reasonable line of route. Tickets can only be used on National Rail services.
Basically if you can get to a London station in the list by National Rail except Thameslink then the ticket is valid.
A few stations can be passed through e.g. London Bridge on way to Cannon Street, Charing Cross, Waterloo, Blackfriars or City Thameslink
In your case I think you can get to all the SWT, Southern or South East (except the High Speed line to St Pancras) terminals if you go via Staines.