The legality section doesn’t repeat what you said above. You made the inaccurate assertion that paying for a prostitute is illegal in the relevant jurisdiction, which it is not. I have no idea why you’re bringing up Northern Ireland and Ireland now, as this event didn’t happen in those jurisdictions.
We have no way of knowing that the prostitutes engaged in soliciting. “Soliciting” doesn’t just mean “finding someone to pay you for sex”, it has a specific meaning to do with advertising for sex in a public place. If the prostitutes met their clients by, e.g., chatting to guys online, then that would not count as soliciting.
Similarly, we have no way of knowing that more that more than one woman actually had sex on the premises, or that the prostitutes have a pimp.
So this is at worst a grey area in terms of legality. And it’s effectively straight-up legal, given that the police have made no attempt for decades to target this kind of prostitution via laws against pimping/soliciting/whatever, since it’s impossible for them to construct a prosecutable case.