Governments gave away power and control a long time ago, and without international agreement to change that, no one countries administration effects them to any real effect.
You are wrong.
We still have near total control of fiscal (spending / tax) & monetary (interest rates) policy. The BoE is independent in name, but it's still paid for and set targets by the government.
I think Greece (which is in the Eurozone, and therefore has less control over monetary policy) provides a suitably vivid illustration of what lax fiscal policy can do for a country.
Ultimately, the issue of a hung parliament causing problems with the markets (ie, our credit rating and interest bill) boils down to fiscal policy. Everyone could see that Labour were deferring fiscal cuts until after the election, and that is why our ratings have stayed steady.
The issue is that people were expecting cuts ASAP after the election, and a hung parliament resulting in a weak coalition will not be able to decisively cut fiscal policy. This means that our outgoings will continue to outstrip our income, we will be seen as less likely to pay up, and Britain will move from borrowing on the equivalent of mortgages to credit cards and payday loans.

