just how much is being lost due to theft, not necessarily an absolute figure but an estimate from and authoritative source?
After just a quick Google, according to this article on the BBC News website, it’s approx £100 million per year in London with the source quoted as TfL:
https://www.bbc.co.uk/news/uk-england-london-47358028
People who dodge paying fares when travelling on London transport are costing the system £100m a year, Transport for London (TfL) has said.
The amount lost to fare evasion across the network is nearly £15m higher than previous estimates.
London TravelWatch, TfL's official watchdog, has called for more revenue protection staff and investment in security improvements.
TfL said there was a 15% rise in prosecutions last year.
This is the first time the estimate of lost revenue has reached £100m. The last estimate in 2016 put the annual loss at £86m.
Fares go unpaid in a range of ways on London's transport network, from failing to touch in and out with an Oyster card to the use of counterfeit tickets.
Some 450 revenue inspectors are employed across the network to try to stop fare dodging.