Jeff Layman wrote:

Sadly, there is little if any correlation between "a few degrees of frost"
in the USA and what we might experience in the UK. The major difference is
what happens in the summers, if that doesn't sound too odd. *Over there,
they often experience summers with many days above 35°C - sometimes above
40°. *That apparently hardens the wood of certain trees and shrubs which
allows them to withstand winter cold better than here.

Even when you get Bauhinias to over-winter successfully, they need hot
weather to flower well. I've had B. yunnanensis growing here for
about 10 years and it is an indifferent performer unless we have a
prolonged hot spell. I completely gave up with B. purpurea, which
survived winters well but only gave the occasional flower right at the
end of summer.