They really are OPI and there have been posts on moneysavingexpert.com about people finding them in Poundland stores and 99p stores. Mostly shatter/crackle ones but someone found the entire Spiderman collection in their local Poundland!
Poundland is American-owned and sometimes high end American brands end up in there like Stila recently but Poundworld is British owned, so they probably won't have them.