When it can't meets its debts as they fall due without printing a load of money?
If that't can't ever be the case how about you tell us. It would look a lot less troll-ey than just taking the píss.
I tried to engage (in good faith) with that fella yesterday on the same topic. I got a rant about fractional reserve banking (doesn't exist in the UK) and Cyprus ( a country which uses a foreign currency) followed by abuse. I'm happy to discuss with anyone who wants the discussion until/unless it becomes unproductive.
Printing money is a misleading term, govt doesn't print much these days it's pretty much digital. Whenever govt spends (buys things etc.) which is pretty much every day they instruct the BoE to create new £s via keyboard.
Here's a simplified version of govt fiscal ops - injection wise:
For example, if govt had a contract to pay you £1,000 or whatever tomorrow then, simplified, HMT would instruct the BoE to mark up your account by £1,000.
BoE, by law, has to comply:
. it doesn't check for pre-existing balance (or is there enough money) in govt 'current account'
. it doesn't check what tax revenue might or might not be coming in that day - it doesn't know anyway
. it doesn't check how many bills/gilts HMT might, by choice, issue later that day - it doesn't know anyway
. it marks up your account and marks down the govt 'current account'
If parliament has authorised the spending, regardless of purpose or size, then nothing can legally stop it.
That's how UK govt can always meet any bill denominated in it's own currency.