Antifreeze Protection

[Saff]

Hi all. I've been measuring my coolant antifreeze protection level using a refractometer and found that my car's (presumably G12) coolant protects down to -16 degree C. Is that low enough? Granted I live in the rather sheltered North West of England and wouldn't expect temperatures to get as low as that but I was wondering what everyone's thoughts were. I also tested Mrs. Saff's Mercedes coolant and that was ok to lower than -40 degree C.

Any opinions folks?