In reality they can stop it as the technology is there and is being used by the goverment, but it would mean the ISP's inspecting your data and using this to determined any illegal downloads, even SSL won't get past DPI as only the content in encrypted and not the location/address...
But as our legal framework stands no ISP is allowed to do that though if the BPI do make a stand any legal victory by them will take the ISP's a step closer
Kymmy