Halfords sell lightbulbs and accessories - they ARE NOT vehicle technicians.
It would appear that they left your car running whilst deaing with the tyres ?
You need to find where the coolant is going. It's either being burned, evaporated, or it's leaking. The spark plugs will show evidence of discolouration if it is eing burned. The leak test to which you are referring pressurases the coolant system to operatnal pressure. normally approx 20 psi max, this will force out any coolant to help you to physically 'see' any leaks. If the system is open to atmospheric pssure ( ie a leak ) it will boil at a significantly lower temperature and simply boil away.
The correct 'leakdown' proceedure will see the removal of the spark plugs in turn and a stethoscope used inserted in the hole to listen for headgasket / cracked liners etc ... with a tell tale * pssst *
Like i say - Go and buy your bulbs from Halfords....