One of the many problems with this is that, like your credit rating, you will never know just what information is held about you, nor when it is looked at or who looks at it or what it is used for.
If you were working in the local tax office and your neighbour's name came up on your screen, would you a) ignore it, b) just get the information you need for tax purposes, or would you also c) look at the websites they have visited and their Facebook page? Then what would you do with the information? Maybe this is what the government intends - that we should police ourselves.
But also the fact that you had not visited any suspicious websites could be latched upon by the Defence in court as evidence that you are not a terrorist/paedophile, when in fact you are.
So many ways in which this could be misused, abused - so much scope for human error, even without malice.