Buggies and prams using the space can be frustrating. The driver can only request that those users vacate the space. They cannot force it. You are right, wheelchair users do have priority, hence the driver is required under law to request the space be vacated and explain why, but if passengers do refuse, then the driver isn't breaking the law as they have done everything they reasonably can. The ramp not working is frankly not acceptable either way.

I'm pretty sure that almost all buses only have one actual identifed wheelchair space. The space on the other side can, in theory, accommodate another wheelchair user on most vehicles, but it doesn't have the supports and accessibility to the polls etc, so many drivers will not accept a wheelchair in this section, simply on the grounds of safety. After all, in the event of an accident, a wheelchair user who is hurt whist travelling in this location, could have a liability claim.