The 1st number (0 or 5 in these cases) is the base viscosity of the oil, and the 0W-rated is good for use down to nominally -40C at cold cranking temperatures, and the %w-rated oil is good for use down to nominally -35C at cold cranking temperatures: ie it lubricates and will flow at those temperatures. As neither temperature is expected in the UK, you can therefore use either rating. More important is the '30' rating, which is the rating of the oil using VI (Viscosity Index) improving additives to assist in maintaining the low temperature performance of the base oil throughout the higher engine operation temperature period as the base oil thins with temperature increase. Go higher than a 30 viscosity, and you will lose engine responsiveness and economy and inhibit cooling benefits from the oil: go lower and you lose the oil's ability to support the bearing loads.
So, basically, 0W or 5W really makes no odds in the UK. The '30' bit does matter.
