The Labour Party would have taken a different economic line to the Conservatives, they almost certainly would not have subscribed to "austerity".
The other thing they definitely would not have done was to have held a referendum on Brexit. The outcome of that decision, of course, knocked 2-3% at the lower end of estimates and 6% top end off GDP.
Quite a lot of money to direct to whichever area of spending a party might want to focus on.
So in the absence of any more money pre Brexit, you are suggesting the Labour Party would have borrowed significantly more? And the effect of this on GDP is an unknown so while I agree we might have avoided Brexit, we can't say what money the Labour Party would have had to spend.
GDP with comparable European countries since Brexit doesn't seem to have diverged as much as those Brexit estimates would indicate.
I think the Labour Govts of 1997 - 2010 did a great job on increasing budgets in the NHS, and that commitment as well ad the money itself helped improve satisfaction levels.
Whether they called it austerity or not post 2010, the money wasn't there so my suspicion is that their spending would have been closer to 1997 - 2000 rather than 2000 - 2010.
Given the current historically high rates of tax, and the overall approach on spending outlined by Labour, I don't see there being a significantly different increase in health spending for the majority of the the next parliament.
I think that govts have far less control of the economy than they like to say, well at least when it's going well. I'd hope that in an alternative history scenario the figures under a Labour govt for satisfaction would be better but I'm sceptical that the entirety of the huge drop is just about spending.