Most UK emergency lights are not designed to withstand prolonged periods of extreme cold temperatures. An external emergency light could be installed where no sunlight is ever received. Receiving direct sunlight benefits the emergency light with the suns warmth during the day. The emergency light will generate some of it’s own heat from the control gear and the light itself, but not enough to combat many days and nights of sub zero temperatures.
The component inside that will suffer the most is the battery. Batteries are designed to withstand short periods of freezing temperatures and still provide three hours of emergency light. This is one of the reasons that the batteries need changing every 3 to 4 years. To ensure that the 3 hour duration is achieved. But when it has been exposed to low temperatures over and over again, that degradation is increased and the battery may need to be replaced sooner.
Even though a full duration test may not be scheduled for a few months, it is a good idea to do a test early to see if the batteries can still provide you with 3 hours emergency lighting.