I had a quick look at the Budgets for European ATC and FAA land and the FAA cost about 1/2 the amount per IFR flight or per IFR flight mile as Europe (and the UK costs on the high end of Europe). Within that cost FAA land provides the 'free' flight following service.
A key difference in the 'Free' concept is that in the US the fuel tax is hypothecated to aviation where as it is viewed as general revenue raising not related to the activity in the UK - So the only thing that is viewed as 'paying' is a direct invoice to the end user. (I know NATS and all other ATC service providers don't get a pound from the fuel tax). In addition, WP is correct that the US government views an element of ATC and airport infrastructure as a Public Good that should be paid for through general taxation - so to that extent is subsidised.