Good luck enforcing that! And should a train leave able-bodied passengers behind because the wheelchair space has to be kept free on the off-chance of someone wanting it later?
But even if the wheelchair space is left free, you can get a lot of passengers all around it, in the vestibule, etc, who all have to detrain to get a wheelchair on, then reboard. During this they have to go onto the platform causing obstructions and other hazards. It isn't ideal, there is no easy solution.
The fact is it's a legal obligation that the wheelchair space must be available for a wheelchair in the event they are ready to board and one isn't already there. Fact. And this has been proven and there's case law relating to the wheelchair user who couldn't board the bus due to it being full with pushchairs. And he successfully sued the bus company I believe.
The staff will make a reasonable effort to enforce this on behalf of the wheelchair user. Even if the train is full on arrival, if customers are standing in the wheelchair space or luggage is stored there they will ask passengers to kindly make space please. If people have to step off to make space for the wheelchair user to board then fine. People can then board when the wheelchair user is situated in the dedicated wheelchair space.
Any customer who refused to move from the wheelchair space would be politely informed that it is legally for sole use of a wheelchair user when required and point to the sign above the space which says 'you must vacate this space for a wheelchair user', which backs up this claim.
Anyone who refused further would be told a little more firmly that they need to move from the space if they expect to travel.
Now if someone still refused after all that, and i dont think the other passengers would give them (the awkward passenger) their support by this point, then the guard could either hold up the train to pressure the passenger to do the right thing or would potentially have to apologise profusely to the wheelchair user and explain that they sincerely tried their absolute best to vacate the space and disruptive passengers are refusing to move and it's likely the wheelchair user may feel the guard has done their absolute best within their enforcement powers and await the next train .Then claim a delay repay.
Should non wheelchair passengers vacate the wheelchair space if a wheelchair wishes to board from the platform? Absolutely yes. And I think this is probably TOC policy across absolutely all the tocs. The guard can't physically remove anyone but they can ask politely, more firmly and can inform passengers of the legal right of the wheelchair user.
If they've exhausted all reasonable methods to move the standing passengers by instruction then the guard will have genuinely done their best. And the fault then lies with the disruptive passengers who are rudely refusing to move. The wheelchair user will have recognised the TOC staff have done all they reasonably could and I don't think they'd blame the staff if they genuinely tried their best.