There is nothing in theory to stop it going in.
Well, where/when I learned to fly a microlight I was taught controlled airspace is closed to us, but that may be country-dependent.
And of course, as you said, a controller can always refuse entry and there is little to do against it, certainly not during the flight. At best one one could question the refusal afterwards, but what good could come of that?
Is the UK so different
Why yes, that's why I will periodically grin at "just one more UK oddity" ...
wasn't there something with SERA and unified airspace
Yes, but at first it doesn't change much in practice - countries are at liberty to file exceptions and most have done so, to various extents. The one advantage of SERA is that the exceptions become exceptions, I reckon they are supposed to disappear over time.