I know I said in my earlier post that you can't expect a guaranteed speed with consumer ADSL, but 200kbps is ridiculous, and it's barely "broadband" in any popularly accepted sense.
I do think there's a potential breach of contract here, and you should ask them to provide you, in writing, with a firm timescale in which they intend to put things right (not just a vague estimate). If they say they can't or won't do that, tell them that you'll wait a reasonable time (say 1 month as per Gooner5's post), and then if things haven't dramatically improved, you'll be offski and they won't get another penny out of you.
If you took advantage of the 30-day "trial" period offered by 02 when you first signed up, and you were getting a consistent 5mbps throughout that period, you have a good argument that you consequently agreed to the 1-year contract term on that basis, in the reasonable expectation of that service level being maintained.
If you want to quote the Ofcom code of practice at them, that's fine, but I think in any case you're already well covered by existing consumer law (which is *not* a voluntary arrangement as far as they're concerned).
To be honest, if you did decide to terminate the contract, under these circumstances I'd be very surprised if they tried to make life difficult for you.