Granted, elements of the UI are lifted (licensed?) from Sibelius/Avid, but the fact remains that Photoscore (along with NotateMe) is developed by Neuratron Limited, which is wholly owned by Neuratron Group Limited (99% if we want to nitpick - I’m not sure who Organum Ltd are but they own the remaining 1%). Neuratron Group Limited is owned by Martin Dawe (85%) and Zara Dawe (15%). This is a matter of public record - it’s freely available from Companies House.
See here and here.
P.S. You can buy NotePerformer from Avid, too, here. I happen to know that there’s no difference between “NotePerformer for Sibelius” and perfectly ordinary “NotePerformer”, as I’m using my purchase from Avid with Dorico right now. Avid doesn’t own NotePerformer either.