I went to the ODM (Origin Destination Matrix) data and other ORR data in an attempt to get a handle on the revenue protection problem:
There are currently 347 stations with ticket gates, according to the ORR.
The ODM suggests that 26.64% of all rail journeys are between these stations - they are gated at both ends.
A further 23.81% of journeys come from non gated stations and end at gated stations.
So, if we employed smartcard tickets with standalone validators at all non barriered stations, we could fully confirm roughly half journeys with a barrier ticket check using only existing barriers. No validation, no valid ticket.
If we added barriers to 145 additional stations the fractions rise/fall to 65.26% and 14.7% respectively. (EDIT: Note that the ODM data is not perfectly symmetrical between pairs of stations)
EDIT:
Turns out the ORR Ticket gate data was from 2019, so it considers Crossrail to be non barriered.
However, they get barriered the second my programme starts running, so the +145 -> 492 stations for 65.26/14.7% figure is still accurate.
EDIT: Manually added some Crossrail core stations in as barriered, now 33.15/24.29%