OP, I think your healthy anxiety is preventing more rational consideration here- and I don't mean that unkindly. Googling is likely to fuel your anxieties and you need to be very careful about where you get your information from. I hope you are getting the help you need to deal with your anxiety.
I think you are unclear regarding specifics of screening. There are pros and cons to any screening programme (and any other investigation, medication or treatment for that matter). There are criteria to help guide whether a test is an appropriate screening tool or not- known as Wilson-Jungner criteria. I won't detail them here. No test is 100% accurate and will have false positive and false negatives- in the former leading to investigation of people without disease and in the latter missing a diagnosis in people with the disease. Decisions regarding when to start and end screening is based on when it most likely to be prevent the greatest burden of disease without leading to over investigation of people who are unlikely to have any disease. This focuses screening programmes in those age groups who are most likely to be affected.
There has to be a cut off point somewhere and that can mean, tragically, that people who could have been diagnosed earlier will not be- but realistically unless you screen everyone for everything from the minute they are born until they die, this is inevitable. In fact even if you did all the screening the false negatives would still mean some would be missed! And we don't have a screening test for everything anyway. However, the over-investigation could well lead to medical complications for many more people than is necessary or acceptable. And the cost would be astronomical, for very little gain in statistical terms.
And yes, cost-benefit analysis plays a role- it has to as the NHS cannot fund all things for all people. It has to use the money it has to best benefit the whole population. Not just people with cancer- after all cardiovascular disease, stroke and dementia are leading causes of death in the UK across all age groups ( exact breakdowns are different amongst different age groups).
More specifically, we are now using the FIT (faecal immunochemical test) which is more accurate that the previous test.
I would not support colonoscopy yearly for everyone over 50, incidentally. Colonoscopy is an invasive investigation which is not without risk. The burden on GI services would be crippling and would lead to money being diverted into this from other areas. That cannot be done lightly, and there would need to be evidence of a significant improvement in cancer detection AND reduction in mortality before it should be considered. It's not my area of expertise, but I don't think the evidence for this is conclusive. I'm not denying that using colonoscopy as screening may pick up people the FIT test would miss, and that this is tragic for those individuals, but you have to come to some sort of reasonable balance.