Even if totally compliant with regulations, the work may have been undertaken 'illegally' (i.e. without notification as required by law) - but the OP doesn't seem even sure that the work was undertaken 'illegally'.My "call the police" was a tongue in cheek comment! There are not "illegal installations", just non compliant ones.
As for "illegal installations", I suppose that one which didn't comply with Part P (or other legislation) (i.e. dangerous installations) might be regarded as that - but, again, we have no reason to believe that the installation the OP is describing is dangerous.
Kind Regards, John
Yes. So the installation may have been INSTALLED illegally, but is not illegal in itself.