[jxub]

London: probably 25 miles for £1.50 (https://www.timeout.com/london/blog/how-far-can-you-go-with-...) or one of the rare £1 EasyBus tickets from Paddington to Luton which would cover around 37 miles.

The second one is not really public transportation though, and we're getting dangerously close to the £15 Ryanair flights territory.

[Symbiote]

A London Underground fare, off-peak, avoiding Zone 1, is also £1.50.

So, Heathrow Terminal 5 to Epping should be a contender. It's around 28 miles.

(And being run by a private company can't exclude MegaBus, since almost all public transport in the UK is run by private companies.)

[wopian]

Yup Terminal 5 to Epping is possible for £1.50 (2 routes bypassing zone 1). Else its £5.10 direct through zone 1.

[kimburgess]

Potential competitor: North (Gympie) to South (Varsity Lakes) on Brisbane Australia’s ‘inner city’ train network. 260km (160 miles) for AU$20 (£10.50) - cheaper off peak or with concession. Buses also extend this further in each direction using the same ticket.

[ouchjars]

Sydney trains have a more forgiving fare structure for this purpose. Scone to Bomaderry or Goulburn is at most $8.86 for over 300 km straight-line distance.