AdamFrisch:LARS units used to be paid in this manner, but different units got a different amount of money for the same type of service, (eg MOD units got a standard MOD rate while civil units often negotiated a much higher amount for the same service) so the system was revised so that units instead got paid on a standard rate for the hours during which they offered LARS eg mon-fri 9-5, 7 days 9-5 or H24 (that's not a complete list by the way). LARS units have to submit stats to show any closures of LARS during the hours they normally got paid for, and number of aircraft worked.
There is only a limited total amount of funding available for LARS from the NATS En-Route budget (about £2M pa I believe) which in turn comes from Eurocontrol En-Route Charges, and it has to be spread very thinly over many units.