Putting more layers of privatisation between the government and the service it provides will decrease the quality and increase the costs. Sewage is now in our rivers and we pay more for water. Trains are obscenely expensive compared to Europe. Energy is expensive. All of these things cost boatloads because we sold them off and drag the rest of the economy down with them. We wouldn't be spending so much today if we'd invested more in the past.

And private spending is something like 200%. GDP is a measure of added value, not total spending/revenue.