I've seen C. montana rubens thriving in Providence, RI, around zone 6B.

All montanas died for me last winter. I'm in the warmer part of zone 7.

Tom - it will be fine pretty much anywhere in the UK. Maybe that's why you have found no reference to minimum temps - it's not an issue here. They're just assumed to be hardy. Where are you located? (BTW I just Googled 'C montana rubens minimum temperature' and immediately found a reference giving temps in F and C)

Here is a link that might be useful: C montana v. rubens