I've already posted elsewhere on the level of debt in the UK economy now compared to the historical figure - whilst the debts are large they are neither unmanageable nor are they without historical precedent.
I thought we'd established that the real debate is about the deficit, not the debt

Secondly to say the government ran up huge debts is disingenuous - previous governments had failed for twenty years to renew essential infrastructure - infrastructure for health, welfare, and the long-term viability of the economy. Much of the spending that took place in the last decade should have taken place in the previous two but didn't.
As far as I'm aware, much of the spending has gone on salaries/running costs. The core deficit is about running costs, almost by definition.
I don't think anyone would disagree that we have been in a credit bubble - indeed I've posted many time that I was alerted to the effects of that bubble because the housing market reminded me very much of the Lawson boom - rampant prices and silly multiples of income. This was a trans-national policy affecting many westernized economies and from which no UK political party looks clean - Cable called it correctly at the time as I recall but the policies of the then government were part of a wider consensus.
I'm coming to the conclusion that the only way to control it in a globalised economy with mercantilist players, would have been to have capital controls. i.e. to partially withdraw from that globalised economy. Perversely, the capital controls would have been to keep capital out of the country. That would have starved the asset price bubble of fuel.
Given that capital controls were politically impossible, a responsible government would try to counteract the bubble -- most likely by running a Keynesian surplus to remove demand, with the intention of re-injecting that demand during the inevitable bust. Brown's stated "golden rule" was to balance the budget over the economic cycle, though given that he pretended to himself that he'd abolished that cycle, perhaps it's not surprising that he blew the golden rule, too.
Debt is of course a call on future economic activity - however whether that is a call on my activities or on someone else's depends on multiple factors - not least the period of repayment. It also depends on what that debt was called into existence for and whether the product of that debt outlasts the debt itself. In simple terms, if I repay a debt within the lifetime of the good purchased then this is a productive use of the money - regardless of whether I pass on the debt to future generations.
We're back to the distinction of borrowing-to-invest and borrowing-to-consume. Largely, I think New Labour borrowed to consume. Where they borrowed to invest, they did so inefficiently via PFI, which is going to cost more in the long run (and which isn't included in the core debt measures they liked to use ... the whole point of PFI was that it could be kept off the sovereign balance sheet, making the numbers look better than they really were).
A good example of this would be the correct decision of the 1945 Labour government to set-up the NHS and welfare state. Atlee could have directed his government's energies to paying-down war debts - instead he had the foresight to invest in the NHS recognising that although it would take far longer to pay-off the debts, the benefits of the health service far outweighed the choice of economic activity for repayment.
I agree; he had the advantage that the post war boom was about to happen. In that circumstance, borrowing to invest in the NHS was a good choice. If he'd been faced with an energy/resource crunch and global contraction, it might not have been a good idea.
Today, borrowing to invest in our energy independence would also be a sensible thing to do. Borrowing to pay the ongoing energy bills, is not.
As you correctly point-out debt is not of itself an issue for a government provided it can manage to service the cost (indeed this is the case for all debts). Our debts are not due for imminent repayment and the IMF recently commented that there is sufficient room within our economy to raise our borrowings by a further 50% before they would cause concern1. Unlike the Greeks those debts which we do have are not due for renewal until after the next election.
It won't take long to add another 50%, unless the 13% deficit is brought down sharply. It's speed and direction of travel, not the size of the supertanker, that gets you in trouble.
You have described how a Catch 22 would damage the economy but given no evidence for how that relates to our current situation. Given the risk of a disastrous double-dip I see no strong case for the coalition's policies. Darling's policy of reducing the annual deficit to 50% over the lifetime of this parliament was perfectly sound. Indeed most thinking people would recognise that the debt level argument is a stalking horse - time for the coalition to come clean.
"Please, God, make me good, but not yet."
The sound policy would have been to run the economy more sustainably all along. New Labour's track record does not give me confidence that they would have been able to deliver on deficit reduction, nor am I persuaded that "50% over the lifetime of the parliament" would have stopped us breaching the IMF recommendation you mentioned, particularly as Brown obfuscated the true debt position so effectively.