The UK national debt is all in Pounds.
But whether or not that is an advantage is hard to establish, that all depends on how much of an export position the UK will have post brexit.
The UK can inflate its debt away, that is always a nice option to have. Greece couldn't do that, and that was a major reason why it was being crushed.
If their debt is not in UK Pounds, it will limit their economic flexibility severely, and I think could lead to a default if things go bad (which we are currently quite far away from right now.)
Thus is the UK debt in UK Pounds? Or is it mostly denominated in Euros, Yen or USD?