Barry Salter wrote:

And, for completeness, the current list of stations valid for
cross-London transfer on a through ticket (displaying the "Maltese
Cross" marker, is as follows:

Aldgate Embankment London Bridge
Amersham Euston Marylebone
Baker Street Euston Square Moorgate
Balham Farringdon New Cross Gate
Bank Finsbury Park Old Street
Barking Greenwich Paddington
Blackfriars Highbury & Islington Queens Park
Blackhorse Road Kensington Olympia Richmond
Canning Town Kentish Town Seven Sisters
Cannon Street King's Cross/St Pancras Southwark
Charing Cross Lancaster Gate Stratford
Ealing Broadway Lewisham Tottenham Hale
Edgware Road Limehouse Tower Hill
Elephant & Castle Liverpool Street Upminster

From the above, is it legitimate for a passenger travelling from, say,
Aylesbury to Southend, to use LUL between Amersham and Upminster, or are
passengers required to make maximum use of National Rail services?

--
John Ray