Back in the old days, you would go in to the corner shop, pay 60p for your copy of Razzle and no one but yourself and the friendly shop keeper would be know. Now, you access a grot site and it is recorded on all manner of data files and anyone with the technical nous or in collaboration with your ISP can look it up anytime.
But it doesn't, certainly not with the majority of ISPs. Most do keep basic header data for a short period (30 days iirc), as this is required under the data retention acts. However anyone still needs a court order, to get this information and once it is gone, it is gone. Things like who has an IP address and when, is usually kept for a lot longer.
Do you honestly think that this is not happening already? The necessity to obtain court orders has never stopped those that want this information. My guess is that the government agencies (including the police) who already make use of this information would like to make even more use of it and therefore are seeking to make it legal.