Out on Limbs

Why is it that when people do impressions of the British accent, they always use this phrase? Is there something I'm not aware of, like, this was a popular skit on Monty Python...or an old saying from back in the day? It just seems a bit odd that there would be this one seemingly universal phrase used for the impressions, for no apparent reason (that I'm aware of). Brits must hate it as much as the French must hate being called pussies.