You've very probably already checked this, and anyway maybe subsequent legislation means it can't happen now - but some years ago, a friend of mine went for something similar.
When he came to sell it, it transpired that the net rental return he'd been getting each year during the guaranteed period included an element of the original purchase price effectively being rebated by the developer. Maybe this element was mentioned in the small print - I don't know. Anyway, once the guaranteed period was over, the return he received - and hence the capital value - was rather less than he had expected.
I've never bought anything like this, just because a yield of say 10% doesn't seem to me to be enough to compensate for the lack of liquidity and all the risks (occupancy rates, damage by tenants, repair bills etc) which are likely to attend this sort of thing.
To add to AJW's comment - I'd also talk to local letting agents etc, to see if the net return fits with gross rents etc, and maybe any thoughts they've got about new capacity coming on stream e.g. new halls of residence for students.
Just my 2 cents.