When I lived in Northern Canada we used to put methyl hydrate in with the gas (petrol to you) when it got cold. It would mix with the water in the bottom of the gas (petrol) tank and create a mixture that could be burned by the engine. Never had a problem with it even below -50 Fahrenheit. People in Canada still use it as 'Gas Line Anti-freeze'. Its also known as methyl alcohol or methanol.
Beware a little goes a long way! An ounce would be enough for 15 litres, although if you put in more it will raise the octane level. You will have to unthaw the tank first. Once in it will prevent any more freezing.
You will also have ice particles in the carb drillings as well , these often accumulate to form a blocked jet.
When you draw in freezing air ,fuel will not evaporate as well . It can lead to the engine stalling /coughing like a 60 a day smoker, the not so well atomised fuel can also lead to dilution of the oil on the cylinder walls .
Racing mini back in the 60's & 70's used to have a coil of electric fire element type material as heaters directly above the carb to help the evaporation and warm the air for better combustion. It also led to a few flamers as well.
I have played with plant of all sorts and vehicles in temps down to minus 27 o C
Any petrol probs saw me totally clean all the fuel system out I'd fully strip the carb and put all the bits it in a warmer environment enough to thaw any ice ( hot water ?) then use heated dry compressed air to blow the fuel lines , drillings , tubes of the carb as well as the jets ,filters and all piviot points .
I'd the reassemble it ,lub the friction points and reuse the petrol after running it though a 100 squares to the inch brass filter screen .. it is quite surprising how much water , gunge and water soaked gunge come out of canned petrol .
WARNING
Be very careful playing with fuel in these sub zero temperatures .. minus 12 o C petrol will give you almost instant frostbite and as you remove a glove or fuel soaked clothing etc strip the skin & flesh off your hands & body including your John Thomas , for it is still able to evaporate and drop the temp even lower at the evaporating site .
I forgot about using methanol as it was supposed to already be added to our fuel .
It is also used in /with trucks air brake compressors systems to prevent the brakes freezing , so should be available in the UK