I studied the market before changing, having been with Pipex since they were "excellent", right through "average" to "awful".
For me, the right answer seemed to be UKFSN. They're an Entanet reseller, and have excellent ratings on the review websites.
There are ISPs that clearly should be avoided at all costs, and others that are pretty good.
More info
here.
That site is sponsored by some ISPs, but the ratings do seem to be based on real-world customer responses. What they say about the ones I know "rings true".