Rather than being your usual patronising self, how about explaining what I supposedly don't understand? Shouldn't be too difficult for a chap with a first class mind.
Issuing paper on a triple A credit rating, that the UK currently has, is an awful lot cheaper than issuing it on a lesser rating given the relative spreads.
This means that it is more expensive for a country like Greece to issue paper than the UK - i.e. to raise funds - because it is less credit worthy.
The credibility of a country's rating will always be contingent on its government. The idea that Corbyn can come into office and nationalise close on £200bn pounds of private assets, running a budget with a massive deficit as a result and also ask the BoE to engage in QE for the public sector as well as for the banking sector puts that rating at severe risk and once you start going down that route it is very difficult to climb back out because people will only lend to you on increasingly severe terms.