It depends somewhat on the controller.
On the weather compensated units, once the outside temperature drops below 3°, the boiler circulates water for a few minutes and monitors the flow temperature in order to decide whether to fire the boiler.
Not sure exactly what happens without the weather compensator - it is not described in the manual beyond the statement that 'the boiler has an integral frost thermostat which is designed for protection of the boiler'.
If your pipework is vulnerable to frost, then you will need an external frost stat.