Try and make sure you get propane gas if you want to use it near freezing. That's the red bottles. The butane, camping gas and calor in blue bottles, doesn't work when you get near freezing.
As for heating there are portable gas heaters available, BUT, they can give off carbon monoxide which can kill. Probably OK during the day when you are awake but best not to use at night. You can get a carbon monoxide alarm. Use good bedding and a hot water bottle. There is also an underfloor heater for sale on this site which you could fit or have fitted. That would be ideal.
It would be best to use an EHU site, then you can use a cheap electric convector heater, fan heater or oil filled radiator and leave it on all night.
Make sure you try everything before you go away to make sure it all works.