It's much simpler than that. Just don't buy the pesky things in the first place.

Try http://www.ukfsn.org/ - they use Entanet as their upstream provider (no filtering, as another commenter pointed out). Additionally, you are supporting free software by using them, and unlike pretty much every other they are customer friendly - e.g. if you want a MAC code, you can get one instantly from their web site, without them making you go through multiple phone calls where they try and persuade you not to leave.

Check out their statement of policy:

Statement of policy regarding censorship, Phorm/Webwise and other content interception

Our policy is that the electronic communications of our customers are private. We do not intercept, censor, scan or otherwise interfer with our customers' internet service.

UKFSN does not and will not have any dealings with Phorm, the company behind the Webwise system being deployed by some other ISPs to intercept customer internet traffic. We are firmly of the opinion that the Phorm Webwise system is illegal under UK and EU laws. We also believe it to be fundamentally unethical to intercept customer traffic in this manner. It will never happen here.

There is some suggestion that the UK government would like to mandate some form of interception and possibly censorship. We would encourage all interested persons to make it clear to MPs and the government generally that this is not acceptable.

You're confused because you're thinking in terms of a login system, not encrypted data which you can easily see. (i.e. it's transmitted 'at you')

In simple terms, once you've intercepted some data you can have as many attempts as you like to find the right key to decrypt it.