The very action of putting a staircase in converts the loftspace into a space that could be regarded as a 'habitable room'. In such circumstances, Building Regulations apply. Its perfectly permissable to have 'non-habitable' spaces around the house - most conservatories, of example, are regarded as non-habitable rooms for the purposes of Building Control. As a habitable space, there are minimum standards of insulation and fire-safety (amongst the various Regs) that apply. Though I would not wish you to quote me, I think it unlikely that legal consequences would follow. The Local Authority could in theory condemn the change, and issue some sort of Prohibition Notice - as they might with a dangerous building. But there are hundreds or thousands of properties up and down the land built with attic rooms that do not meet the Regs because they were built before Regs even came into existence. Would it make the property unsellable? - well, quite possibly less-sellable. Any buyer's surveyor would pick up on such a change and comment on it - that might put a buyer off, because the buyer would be asking the same questions as you are asking now. However, the fallback position would always be the remove the staircase again - so hardly a disaster.
However, what's wrong with putting up a decent (aluminium?) loft ladder - this solves the problem, fully-legally and cheaply?