Yes it would, and a 20% tax would pay a big chunk. The article gives the total personal wealth in the UK as ~£9tn, of which the top 10% own ~£4tn. The national debt is £1.37tn (~88% GDP). A 20% tax on that 4tn would reduce the debt to £0.57tn (~36% GDP) - the lowest proportion since 2002, just before the Iraq war.
The 90% tax that you refer to on the other hand (a yield of £3.6tn!) would of course pay the entire debt (£1.4tn) with enough left over to triple the education budget for 10 years (£1.6tn) and buy every man, woman and child in the country a second-hand Ford Fiesta (~£600bn).
How do you think governments spend beyond their means at the moment? I'm afraid it's already mainly by borrowing, and they haven't defaulted yet.
I'm sorry but your first point is utter moot. If someone has all their wealth tied up in property, cars, land, whatever, yet their salary is only 50k a year, where exactly is this extra 20% going to come from? Especially considering they will already be paying tax on any property, cars, land, as well as a higher tax rate than the a lot of the UK. Someone's wealth on paper is completely different to how much fiat they hold to be able to pay, for example, another 20% tax. The only way this would be done if buy taxing earnings even more, which is a complete different measure to taxing somebody's, 'wealth'.
While your right on the second point. Saying they haven't defaulted yet isn't exactly a measure of faith or trust. Both UK and more recently, the US have come very close to defaulting. Another crisis on the scale of the most recent one or worse would, without a shadow of a doubt, bust all major western economy's, and you might not have to wait for this as long as you may think. The gravy train has to stop some when!