Are you saying BTS has a centralized authority that can enforce the continued use of BTS after a major crisis and loss of confidence, until it recovers? That seems to be the only reason people are still 'in' GBP.
No, but in exactly the same way, value doesn't disappear into nothing even in the face of a crisis. If the bank of england had decided to adopt the EUR and drop GBP at that time, that would have forced everyone out. As it was, GBP still had significant value and remains one of the worlds strongest currencies today.