The issue is when there is a monopoly. When you have a government monopoly of a specific industry, it's going to be poor. If the same thing happens in private industry, it's a similar outcome. There is no competition with the government, because they play by rules that makes it impossible to compete as a private business, so it's by default, a monopoly.

When companies compete, it's always better for the customer. Government run Internet (which is what we had before it was commercialized), was stuck in the same place for decades, with no innovation. It's pretty much universal and relatively cheap now.

In the US, you can look at the post office and private companies like UPS and Fedex. I used to run a business for almost a decade where I would ship out 100s of packages/week. The post office always had poor service compared to the rest...and the cost ended up being comparable. If you lost a package? with the post office, there was no real way to get your money back.

There's waste with government-run services because they never have to really worry about profit or losing money. Every government-run service I've ever used has been inefficient.

How much would be enough? 15% of GDP, 20%, 50%?

I'm generally in favour of public services over ruthless privatization but I don't understand how Britain is to survive as a nation of nurses and pensioners supported by a tax base of Uber Eats riders.