LynneJones, 
https://keepournhspublic.com/funding-the-nhs-a-plain-english-guide-to-money-and-affordability/ “Both parties adhere to the present government’s self-imposed fiscal rules, the principle one being that the ratio of debt to GDP must be falling in five years’ time. Any proposed public expenditure is measured against that fiscal rule – what is called ‘fiscal headroom’.”