And what on earth would these oligarchs and parasites live on?
Where would they get enough food, raw materials, labour, products, machinery, money, etc. etc. etc. to survive?
If from only their own working class then surely they would be forced to rebel and would call on all the liberated states to come to their aid?
Do you really think that we would have the same political climate here if we were the only capitalist country left on the planet? Do you think that the working class here is not going to be radicalise in the face of the increased poverty, repression and oppression as resources and profit from abroad dried-up? If anything Britain would be one of the first to be hit.
As other state fell like dominoes the situation here would change out of all recognition well in advance of Britain managing to survive as the last capitalist state. Britain was the first modern imperialist country and it can no longer survive unless it has other countries to oppress and exploit - any more than capitalist countries could revert back to a feudal economy. The key to the revolution in Britain lies initially with those oppressed countries.