The Tories are not paying down the deficit.
I think you have the deficit and the national debt confused here. The Tories have been correcting the deficit (the difference between annual revenue and spend), it currently stands at about £63bn to £70bn from the high of £160bn in 2010.
The national debt has not been reduced (or paid down) because in an environment of deficit, you are still borrowing your shortfall. These borrowing requirements add to the overall national debt that, as a result, increases.
The reason is simple. It's being starved of funds.
You always need to be careful with figures that compare rates of spending to GDP (as in the article you cite) because GDP is a measure of an entire economy. Your economy shrinks or is small, your healthcare spend can look high, and visa versa.
To some extent, you could raise the % GDP for healthcare spend in the UK by just removing money from the economy by whacking up the interest rate. It wouldn't mean the actual NHS spend was higher or better; it would just mean your GDP shrank.
According to the world bank, British spending on public healthcare is currently 7.6% of GDP. What does this actually tell us? Well, considering public health spend in Lesotho is 8.1% of GDP, not very much. Are we really suggesting that public healthcare in Lesotho is better than the UK?
What gives us more of an idea is to look at public health expenditure as a percentage of government expenditure: ie. how much of our annual budget goes towards public healthcare.
Here, the UK spends 16.2%, Spain 15.3%, Italy 14.2%, Ireland 12.1%, France 15.6%, Austria 16.2%, Portugal 13.4%, Belgium 14.9% ... but it becomes complicated to compare because similar principles apply. Private healthcare may make up more of total healthcare expenditure in another country; there may be a mechanism for private top-ups to public health provision that do not show up in the public expenditure stats; such private care maybe cheap; different budget priorities may mean the cost of public healthcare appears high etc.
For example, take Portugal at 13.4% on this measure ... public healthcare expenditure in Papua New Guinea is 13.2% on the same measure. The idea that Portuguese public healthcare is comparable to that in Papua New Guinea is somewhat bizarre. 
Interestingly, on this measure, the overall public healthcare spend as a percentage of government expenditure for the EU area is 15.9%. So Britain spends a greater percentage of its public funds on public healthcare than the EU average.
These are all world bank figures by the way. >> World Bank figures for health expenditure, public (% of government expenditure)
The thing is you can work stats to create whatever narrative you want, which is why you need to be so careful about them. Take the US ... they spend an extortionate amount of government expenditure on public healthcare. It's about 19.5%. But that doesn't mean that US medicaid and medicare provides a better and more comprehensive service to US citizens that qualify for those services than the NHS does to a similar segment of the population in the UK.