The UK won't default in the sense it will fail to pay it's debts.
That's because our debts are denominated in our own currency. And we can print as much of that as we like.
Printing currency does have consequences though, as currency is not wealth, it is a store of wealth and printing could end up trashing the currency. So a technical default as it were.
I think there is a general acceptance that the markets feel that the UK has come to the end of it's credit line, and further spending needs to be paid for directly, in other words raised by taxation. In some ways I think the markets are exercising a greater control over democracy than the voting public (it appears the government(s) can seemingly lie to the voting public, cover up information and not do what they promised as part of their manifestos, whereas it appears they cannot do this to the markets who vote on a daily basis. However that discussion is probably one for a different thread).
And here we find the crux of the issue, it can be loosely categorised as a productive population that feels they are being taxed too much, a non productive population that feels they are under attack and having their benefits reduced, and to complete the toxic mix, a substantial number of people coming into the country that are adding to the spending burden, simultaneously requiring increased taxation for the productive and decreased benefits for the non productive in order to balance the books.
This is the situation the current government has to work itself out of. Not surprisingly the opposition parties are attempting to make political capital out of this difficult situation, the benefits of course of not being in power.
To come back to the point (defaults), every GBP someone holds is a bet that the government(s) (plural includes future governments) can resolve this tricky situation without trashing the currency in the process.