I agree that there’s something rather dubious about the practice of landbanking, buying agricultural land, splitting it up into plots and then selling it on as house plots which might get planning permission. The actual illegality depends upon exactly what people are told and things like whether it is a collective investment or an individual one. However, this number from The Guardian might be worth putting into context:

The CPRE has found 28 companies selling enough land between them to
concrete over the space of the City of London ten times or more.

The City of London is, famously, The Square Mile. So that’s ten square miles. Area of the UK is (sorry, no calculator to do the km to sq mile so a guess here) some 100,000 square miles? Even if we took only agricultural land, it’s 20,000 square miles.

So we are talking about somewhere between one ten thousandth of the country and one two thousandth. OK; it may well be a problem for those who have ‘invested’, but I don’t quite see it as a threat to the purity of rural Britain.