I'm not sure if this is true. There are options for us to improve both controls on individuals and information gathering without dramatic day to day changes for the vast majority of the UK population.
In both of the last 2 terrorist attacks in the UK, the perpetrators were 'already known' to the security services. So we can use these two as good examples to investigate whether there was a failure to restrict, an inability to restrict or a failure in intelligence gathering applies.
It is true that the we can never prevent all teŕorist attacks, but that doesn't follow that we are at the point now where nothing more could be done.