I doubt it’s a stuck thermostat.
If the thermostat was stuck open, it would take an age for the engine to get up to temp, most of the time it wouldn’t even get to normal temp the needle would sit a bit to the left of the centre.
I very much doubt it’s a coolant leak, with the milage you have covered you’d have run out of coolant by now, when the car is running the coolant system is pressurized and any small leak will result in the level dropping fairly quickly.
It won’t be the headgasket either, the engines in those cars are terrible (no offence) but they vary rarely do the gasket even when run without coolant.
I’d put my money on the heater control valve. It’ll be bolted to the bulk head at the rear of the engine bay with two rubber hoses going into it – these hoses dissappear as they go inside the car into the heater matrix. The valves are a common failure on Fords.
Ross.
Mechanic.