'How much do I want it to go up by?' isn't the approach I would take. I would look at what the country needs in order to provide a decent safety net whereby, if you fell out of work or were a drug addict or ill, you wouldn't have to live on the streets, you'd be helped, and have enough to eat, drink, and not freeze to death. I'd also pay for police, judges, an army, prisons and so on and whatever the cost was, I'd say it was a price worth paying for a civilised society. So those who can pay more would pay more - so yes, everyone would have to pay, including the middle class. How do I persuade people to vote for it? Tell them about all the other examples in the West and how they consistently rank the highest in almost all metrics on happiness, education and welfare, and suggest it might not be a bad idea to follow.
With all due respect, you're avoiding the question somewhat. There are lots of things that we could learn from other European countries, such as why they seem to get better health results for similar money than we do. But you did raise the point about France and higher spending, so in order to have plans to do that, we do need to know how much more is being talked about. How else can we know what to raise?
I'm not asking you for chapter and verse on the public accounts here, but a nebulous "spend more money" argument is a bit meaningless unless it's aligned with a "this is how much you'll pay" equivalent. This is actually the worst part of the current Labour front bench, they rack up commitment after commitment without ever saying how it would realistically be paid for. And for good reason too, they know they'd get killed at the polls.