I appreciate that you were among the one third of cases who are not picked up by this test retrocop, and that you feel let down by this.
However at the moment, until better screening is generally available, it is certainly better than nothing, as long as one realises the limitations of the test, and that even if the test is negative, any worrying bowel symptoms should be reported promptly.
I am surprised that your GP did not know the limitations of the test and the failure rate.