I think that these private screening clinics play on peoples fears re health. Best thing is to go to your doctor
if you have symptoms that you are unsure of.
Agreed.

But.. Another way to think about it - if we're talking cancer, at any rate. Not many of the common cancers are slow to develop. I have precious little medical knowledge (next to none, in fact), but, from friends and loved ones who have succumbed to various cancers that have been caught late and treated late, they've lasted perhaps a year to 18 months at the most from initial symptoms (which may not have been attributed to cancer at the time). Obviously, with early detection and treatment the outlook is very much better.
Now, I don't have the medical experience to know at what stage in that process a "Lifescan" would have pickled up anything and how it would have changed the outcome, but it suggests to me that a one-off scan is pointless, as, putting it bluntly, you could still be dead in 18 months. Not really much peace of mind. Now, if it could economically be done every 6 months, and without the side-effects of scans adding to the risk, my observations suggest you might be getting somewhere, but that's not realistic.
Again, no medical expertise here at all, but it strikes me that a scan performed in response to a symptom would be more effective than one done whilst otherwise healthy in any case. In the former case, the consultant will already have an inkling what he's looking for, and will investigate anything he sees on a scan along those lines. In the latter, do you do a biopsy on every shadow you find anywhere in the body? Completely different thing, IMHO.
Do you strip your car down every 6 months just in case it's about to raise a fault code, or do you wait for the code and then immediately look in the area the code points to?
It seems to me that the best course of action is to look after yourself, take note of any symptoms, and have a doctor in whom you have the confidence to refer you if anything suspicious emerges.
With pretty much any other life-threatening disease, there will be signs that can more easily be picked up IMHO. Get your blood pressure, sugar levels, cholesterol, liver and kidney function, etc. checked on a regular basis and you won't go far wrong. Luckily, I have a local surgery who do all that and with whom I'm confident, although my GP has just retired and I'm yet to see his replacement.
Frankly, I think, if you're worrying, it might be because you don't have confidence in your GP when he says you're healthy?