Sorry, I don't agree with this idea that there cannot be clear set instructions from management. If a company want their staff to behave in a particular way and expect any sort of consistency then they need to set the rules, not leave it as a "judgement call" and subsequently second guess any decision made.
OP, best advice for the job in general is to always look out for yourself first in every situation. Don't put your licence at risk by breaking traffic laws, don't put your safety at risk by having conflicts with passengers or other road users, don't put your job or personal reputation at risk by having arguments with them either.
That scumbag that struts down the bus without paying is an anonymous nobody, you are always identifiable. Your manager, control, TfL and the police will likely do nothing at all if he doesn't pay the fare or smashes up the bus or assaults you but any and all of them will have their boot on your neck over the slightest transgression because you are an easy target.
If your company will not give you clear guidance on dealing with non-fare payers then anytime it happens use the radio to contact control and let them make the decision, your options being carry on regardless or turn engine off and wait for suitable authorities to attend.