All the points raised in the comments have already been adressed in France with the debates around HADOPI.
The burden will be on the user to prove he is NOT guilty.
IPs seems to far to be picked at random (amongst french IPs on torrent trackers) by a company called TMG; some gross mismatch have been detected, including strangely enough internal IPs... which shows a high level of incompetence.
The cost will be paid by ISPs, they will raise prices (like they did here)
Next step (in france at least) is to have "voluntary" filtering enabled. A log is kept on your ISP-mandated modem/router of your connections so you can "prove" you are not "guilty". Of course if you refuse this filtering to be enabled you are deemed guilty by default
Some other goals are: filtering direct download sites and VPNs; some people, who might actually be tinfoil-lovers, claim they will also ban SSH on "consumer" grade connections and require a "business" grade connection to get SSH working again. This would be disastrous for the economy here as more and more people work from home using VPNs and SSH.. but it seems the government can't put 2 and 2 together. To them internet is a media consumption network with only 1 way data flow.
HADOPI is already having disastrous consequences for french hosting companies because people don't trust having their machines on such a crippled network.
VPN sales in other countries are surging, of course.
So brace yourselves UK if you government is looking to copy HADOPI this is what is in store for you.