According to the article any non-matching scan is immediately deleted, so that answers one of your points.
Also as I read it they are using it to find people who are either already wanted (and are avoiding arrest) or considered a threat to others aka think of the children approach as given in an example on the article.
As for which offences I would expect that once you have either done your time as His Majesties pleasure and/or paid the fine, has points on your licence then the offence is "spent" as far as in being wanted for arrest and there is no need to be looking for you.
So this does have some positive uses but as always mission creep can set in and before you know it we are in 1984, so yes there has to be some laws laid down soon that limit how the scans are stored (should always be never unless a positive) along with some means of erasing false positives and ensuring that it is really only used for the examples given above.
Oh and as for tracking you around, if you have a mobile phone that is already doing it and the national ANPR camera's keep track of where vehicles are, so we are already part of the way there.