Very true.
There are a number of points here. The thing with Gleason 6 (3+3) is that it indicates a slow-growing cancer, which means you don't want to be bounced into a quick decision to undertake a procedure. Because any procedure is bound to have significant side effects (whether short term if you are lucky, or long term) which you are sure to avoid during Active Surveillance. Private medicine is more oriented to undergoing procedures than long term monitoring.
But... the Gleason score has come from a biopsy sampling a small percentage of your prostate, although the person doing it will have tried to sample all regions and especially those where cancer is reported most frequently. Which means there might be some higher grade cancer cells that were missed in the first sampling but which are picked up in your next biopsy. Or some cells which progess to higher grade. So you as a patient need to be on the ball to ensure all the tests and biopsies actually do take place while under AS, and the doctors respond accordingly. From what I understand some patients are never upgraded so never need invasive treatment, while others find themselves undergoing a procedure after just a few years - but those few years are free of side effects and worth having.
Whether you can extend your time before needing a procedure by changing your diet, as Lizzo suggests, is more controversial. There is some evidence suggesting incidence of prostate cancer is higher in those whose diet includes saturated fats (especially C14, C16 and C18) but that mosly depends on studies using diet questionnaires which are likely to reflect longterm food intake (e.g. https://pubmed.ncbi.nlm.nih.gov/38291432/) - it isn't clear whether a short term change in diet following initial diagnosis would change your risk. But having said that, for anyone worried about their health it makes sense to eat a balanced diet (so probably more fruit and veg and less meat than most British people normally eat) and keep physically fit.
If Mark does need treatment, it is possible that private medicine allows access to some procedures which are not yet standard and available on the NHS - but that probably means the medical world hasn't yet fully accepted their enthusiast's claims about them. And of course it involves more certainty about the consultant doing it since he or she will be wanting their fees. However those aren't a guarantee your personal outcomes will be "better"