If you look at the history of UK debt, it was about average for an OECD country before late 2007/2008, then shot up.
https://imgur.com/ArbLVGs
(this graph is somewhat accentuated by the fact that the UK went into recession in '08-'09 and it expresses debt as a GDP %, but the effect is much the same if you look at raw numbers).
No prizes for guessing why. The bailout which became necessary during the great recession - and the inherent costs of a shrinking economy - vastly, vastly outweigh anything else and we're still living with it. Of course, the fact that we constantly follow the USA into extremely expensive (and ultimately pointless) military ventures does not help at all, and easily outdoes any "efficiency savings" made by shutting down libraries or fire stations or taking benefits from disabled people.
Foreign Aid (which usually comes up at this point) is of course a potential saving, but is a bit of a different discussion which would be better served on another thread. For present purposes though it is effectively small change in the wider context we are talking about here. If the UK's foreign aid budget were zero it would have just the same problems as it does now.