20F is about -6.7 Celsius so covers the majority of the UK:
source
Although I'm borderline (in the yellow coastal zone), we have had -10 or so in the last 15 years since we've been here. I'm running out of potential spaces though....
unless I don't play along next year.
Edited to add - I had a wander round today and think I still have at least 2 areas I won't get round to improving this year, so maybe.....