Having said that, it would almost certainly work OK (though maybe not as well as normal loft insulation), and no one is going to check up on you on the off-chance that you have done this.
But there are some issues you may want to consider:
- Come the time to sell the house, the buyer's solicitor should ask about things that don't conform with Building Regs., so the saleability of the house may be affected.
If the house burnt down, a) the insurance company might try to use this as a reason not to pay out, and b) if somebody was hurt, I suppose it's not inconceivable that you may be prosecuted for negligence of some sort, and possiby sued also.