London is mostly protected for the tube, but they're manned whenever the station is open (and if not, left open). In principle, I think they're ideal. If you knew they would be in operation until the last train has left, you couldn't try to get by without buying a ticket.
And, you then complement these with revenue staff that check tickets on trains; checking people are on the right train, aren't in first class with a standard ticket, or overriding (or indeed, getting a ticket from the station before just to get out the gates). The same staff also provide a visible staff presence and reassurance, especially at night.
For there to be a proper deterrent, you have to do things properly. Once you do, there's really no way for anyone to try it on (and those that don't have a ticket are far more likely to be almost certainly telling the truth).
The problem is that once you do that, you cut fraud significantly and although ticket revenues may increase, that won't be seen as being down to the security. No, that will be down to 'growth', and with no money being generated in fines - all that security will be considered a waste of time 'because there's no fraud'. They'll be cut.
Only the DfT can solve this by making it a requirement of future franchises. Nobody will do it voluntarily.
