A lot of people use non food grade CO2 but I wouldnt take the risk. The Co2 put into the bottles is the same gas but there are far less controls over the filling of the cylinders so you could have many different contaminants in it.
I had a quick look on Hobby weld website, there is a supplier in Whiddon down (15 miles from Exeter). Most hobby weld suppliers do food grade CO2. Not the cheapest but far cheaper than sodastream if no other options available and only about £10 less than others are paying.
I certainly wouldn’t use it, no matter how small the risk “food grade” is food grade for a reason
Adam’s gas are great if you are anywhere near the southeast