As I understand it, the need for UK regulation only came up due to brexit (before they were in thanks to the common market), so I guess "it's brexit" is sort of right, even if that leaves it a couple of steps?

Without it they could have coasted along for a while longer with not much extra effort.

That would confirm what they're saying - that the costs of continuing to give it a go in the UK market will increase substantially post brexit. Inside the EU costs would have been lower so it was worth trying to gain a foothold.