In fairness to Phil, I think he means that whatever was required of him, he did just that and was contacted by Pure Insurance, whoever they are.
Richard (the insurance guru) may want to come back himself, but here are a couple of thoughts from me.
If this were a generalised breach, there would be - literally - tens of thousands of complaints like yours given the volume of quotes that are provided on a weekly and monthly basis. Yet in some three months on this site, this is the first one I have come across. There may a few who will say they too have been contacted in the same way, to be sure, but hardly likely to be that many.
TBH, it logically doesn't make sense that we would "pass on" your details - and place ourselves in breach of the Data Protection Act - yet not do so in tens of thousands of other cases. I mean, if ms.com are really going to screw potential customers and throw away the goodwill they have, all for the sake of a few extra quid, they might as well do it properly, eh?
It is, of course, still possible that ms.com made a genuine error in your case. Yet I wonder whether the problem is actually down to ms.com.
RGB has suggested that maybe you didn't tick the right box correctly. That is a possibility, one which I've done before, confusing ticking wth unticking. But if you say you did everything correctly, that must be discounted, of course.
Alternatively, at the moment when you apply for a policy, it is the insurer who deals with you when/if you leave our site and then try to obtain a quote directly from it. At that point, you are giving that insurer the information, not ms.com. And it is the insurer whom you ought to be telling not to contact you again.
Still, if you really are certain this is a breach of the Data Protection Act, the person you need to complain is the Information Commissioner, whose website is easy to find via any earch engine.