Oversubscribed just means more people applied than there were spaces - and when that could mean in some areas that the parents added them 6th of 6 preferences or as a late preference having not received an offer where they really wanted to go (or not at all once a whole bunch of offers are declined).
I'm also familiar with the content of Section 48 inspections in England, so I know exactly what's required and that it's the decision of the Diocese/Archdiocese whether everybody has to sit RE for GCSE in the case of CofE or RC schools.
Other schools can simply offer it as an option - and whilst the Archdiocese wouldn't like it, it would be perfectly possible for a non Diocesan RC school to decide to not make RE compulsory - it's usually a fairly easy shoe-in for higher grades with a greater proportion of kids from Faith Groups, especially if they are medium-higher attainment in English/Reading, Grammar, Punctuation and Spelling at KS2).
Local authorities in England also stipulate the religious curriculum for maintained schools - and it is always very broad in scope at all phases.
I don't see the point of digging at people for the fun of it, though. They believe, which is the whole point of faith. Ripping the piss out of them for it isn't going to make them suddenly think 'Oh, you're right, it's all bollocks really, isn't it?'.