Hmm.
The Public Service Vehicles (Conduct of Drivers, Inspectors, Conductors and Passengers) Regulations 1990 (the 2002 amendments are around disability access) are relevant bit of law here. (copy
here)
Yes, the passenger in question would be in breach of regulation 6 (1) b if they "put at risk or unreasonably impede or cause discomfort to any person travelling on or entering or leaving the vehicle, or a driver, inspector, conductor or employee of the operator when doing his work on the vehicle"
And under regulation 8 (2), "Any passenger on a vehicle who contravenes any provision of these Regulations may be removed from the vehicle by the driver, inspector or conductor of the vehicle or, on the request of the driver, inspector or conductor, by a police constable."
This is a little bit vague in terms of just what a driver, inspector or conductor can do to remove said passenger, they do not specifically authorise or prohibit physical force, and I can't remember (or find on a quick web search) any case law that might clarify this.
However, even 20 / 30 years ago, the general drill was you ask someone to get off, ask them a bit more firmly, and if they don't want to play, call the police and let them sort it out (although there is anecdotal evidence out there of conductors using ticket machines and, in extreme circumstances fire extinguishers, as defensive weapons)
I can't see why any bus driver would want to get in to a position where they would risk at least being accused of assault and might risk being thumped - or risk other passengers getting caught in a fight. Many bus companies offer advice on dealing with disruptive passengers, and generally speaking the advice is to try and avoid / minimise physical confrontations - I can't see then encouraging drivers to start it and I'm not aware of any including unarmed combat in their CPC training...
Some operators also tend to consider whether a driver has contributed to escalating a situation following an on-bus incident in the same way that contributory negligence can be taken in to account in an accident at work.