It sucks for them. I'm sure they'd be against it. But I don't think we can allow such an important national asset (I'm not an epic lefty... I just mean that we obviously have a finite area of land, etc) to go unexploited, essentially. Obviously they rightfully own it, etc, etc, but I also think they have a duty to actually realise the asset... especially when we have a BIG housing shortage.
I'd suggest possibly saying it's a precondition of granting planning permission that it should be built within x years, or something. Yes, planning permission expires (in all cases?!) but it can be renewed/granted again/extended. But, if we say that when it's granted it must be used, else they're not permitted to reapply, that could help.
Some kind of land tax seems, at first glance, fairly dubious... but I'm sure there's a way of holding unused land undesirable through such a measure (obviously that would only apply in certain areas, etc, so Bob the farmer who has a fallow field without planning permission isn't caught out).
I do find it funny how compulsory purchase orders can be used when, for example, regenerating an area... but the idea of using them against developers with land banks is unthinkable. There's a clear public benefit with each, no? One which arguably justifies the state impinging on the rights of a landowner?