Modern cooling systems often have a vent valve so that air can be vented off when the water when the thermostat is open. The vent valves need to be at the highest point in the coolant system.
With some ingenuity it should be possible to fit a vent valve to an Allard.
Alternatively one way of “bleeding” air from an Allard cooling system is to remove the radiator cap and bring the engine to operating temperature. When all the bubbles of air have ceased, top up the radiator then tighten the cap and o not remove it until necessary. Each time the cap is removed the bleeding process has to be repeated.