Er, who exactly is meant to be cancelling debts?
The UK currently owes so much money that it would need every single person in the country (from newborn babies to centenarians) to hand over more than £34,000 in order to pay it off. That money is owed not only to foreign governments and banks but also to those institutions, such as pension funds, which have purchased UK Government Bonds as investments.
The debt could only be cancelled if pension funds ripped up their interest-bearing bonds, leaving millions of people without their pensions. It would also need major banks, across the world, to rip up their bonds too, plunging them into financial crisis, which would then devastate economies (leading to higher unemployment and greater poverty) across the world.
The UK's debt grows by over £5000 every second. While we need to find ways of addressing that (through increased taxation), there's no way that anyone could ever write it off.