That assumes the law will allow decent ISPs to do that.

From http://www.boingboing.net/2009/11/20/britains-new-interne.html:
"A duty on ISPs to spy on all their customers in case they find something that would help the record or film industry sue them (ISPs who refuse to cooperate can be fined £250,000)."

And the Bill enables secondary legislation to be used to add arbitrary extra powers practically any time the Secretary of State feels like it.

The government did (to some extent) listen to criticism of the way the
Coroners and Justice Bill allowed ministers to override data protection rules if they felt like it, so there's some hope of the disproportionate measures in this being similarly reduced.
http://www.guardian.co.uk/commentisfree/libertycentral/2009/jan/19/coroners-justice-bill
http://www.parliament.uk/commons/lib/research/briefings/snha-05001.pdf
http://www.statewatch.org/news/2009/feb/uk-ml-convention-clause-152-briefing.pdf