Parts of Hertsmere and Three Rivers are closer to Central London than Watford - shall we move these too?
You're quite right - the webpage I got this from has been superseded. Interesting that, on the contrary, you can use Chiltern services all day from Amersham to Marylebone as an alternative to the Met.Actually you can use London Overground all the way to Watford Junction, it's just WM trains and Southern that Freedom passes are not valid on beyond Harrow and Wealdstone
Watford is too small to be a London borough. it has a sub 100'000 population whereas nearly all London boroughs (apart from City of London) have well over or close to 200'000.
The crater left by the required thermonuclear device would need quite a large viaduct over it, to ensure the WCML continued to provide an essential escape route northwards.
A more realistic definition of London than the GLA boundary is the "Greater London Built-Up Area". The boundaries of the Greater London Built-Up Area, and other Built-Up Areas, are determined by the Office of National Statistics based on the actual extent of urban areas rather than where politicians have decided to draw lines on maps.
https://en.wikipedia.org/wiki/Greater_London_Built-up_Area
shows that Watford is comfortably within the Greater London Built-Up Area, which even includes more distant places like Harlow, Hemel Hempstead and Guildford. Some settlements within the GLA boundary are excluded as there is some land without buildings between them and the Greater London Built-Up Area.
Built-Up Areas are also more sensible terms of reference for many towns where the urban area extend well beyond the municipality boundary, such as Reading and Nottingham.
To be honest, I would add Rickmansworth, Bushey, and Oxhey into a newly created "London Borough of Watford".