HMRC does not take security as payment of taxes. Security implies risk and HMRC does not take on risk. HMRC deals only in cold hard cash, so any talk of putting up security with HMRC is wrong. Any other punter has to take their security to a bank and realise cash and then pay HMRC with that. That's what banks do, they take the risk. It could be that this is what was meant, but it's not very clear.