Peter Smyth wrote:

So which of the 5 routes from Dartford to London would you use to calculate
the fare?

Surely that depends on the route the passenger wishes to take?

Table 200 of the NRT gives the following mileages for London to Dartford:

London Charing Cross - Blackheath - Charlton - Dartford is given as
18.75 miles.

London Cannon Street - Greenwich - Woolwich Arsenal - Dartford is given
as 16.25 miles.

London Charing Cross - Blackheath - Bexleyheath - Dartford is given as
17 miles.

London Charing Cross - Lewisham - Hither Green - Sidcup - Dartford is
given as 17.25 miles.

London Victoria - Peckham Rye - Lewisham - Bexleyheath - Dartford is
given as 18.5 miles.

This gives an average of 17.55 miles.

Cheers,

Barry