There's an inflection point, sure, where you start spending enough privately that the insurance is worth it, but so many of these things are so cheap here anyway that the gap from between where it starts to pay off until you're ill enough to get reasonably fast treatment with the NHS is quite narrow.
Sometimes people think it is wider than it really is because people want and push for treatments that are not medically indicated. E.g. patients pushing for pointless MRIs is common enough that same-day MRIs is a huge industry here even though they only make an impact on outcomes for very specific symptoms.
For instance the UK have very bad outcomes for cancer because things tend to be caught late and treatment delayed thereafter. Frankly, for anything potentially serious or time-sensitive I would go straight private (and in fact the NHS tells you to do that when they ask you if you have private insurance).
The NHS has much bigger problems than funding. Even the new, left-wing government has indicated that they won't increase funding without reforms.
It's always very convenient to starve something of funding for years when you want to insist something needs to be reformed before you can spend as much as it costs to provide service at an adequate quality.
Reform or no reform, outcomes won't improve without increasing the funding, as no other system in a comparable country manages to deliver more at the NHS cost level.
Another issue is that the NHS is a religion. It is blasphemous to suggest departing from free-of-charge delivery or private involvement (although that's already what we have). Even suggesting "reform" is badly received.
I'm from the US so excuse any oversimplifications, but over the past couple decades I've noticed a trend of US-ification in British politics. I would be careful with looking to greener pastures.
The French system is based on a mandatory health insurance, now with mandatory private health insurance in addition to that.
GPs and most health professionals are private practices that set up shop like a, say, lawyer would and there is big private sector involvement up to hospital level.
I think there are many similar examples throughout the world.
This is partly why I find this focus on the US system in the UK puzzling. My best guess is that it is used as a scarecrow by those opposed to any changes to the NHS.
Waiting times are not only for non-essential things. The only thing with no waiting time is A&E if you are in immediate danger of death.
The NHS is some "safety net" that does the bare minimum at this point, or no longer anything at all (eg. dentistry).
Interestingly, nowadays if you have a good job and private insurance in China you get better routine checks than in Europe/UK. Even Chinese hospitals are extremely thorough and quick compared to British ones (at least the main Chinese cities)