If that's the case, then there's no need to store the fingerprints in perpetuity, right? You can just delete the record pretty much immediately after the comparison is made. Also, there's no need to take a full set of prints. One or two fingers should be plenty to tell whether or not someone matches a single, already-known set.
So that's two simple tests to see whether the UK Border Agency is telling the truth here.
Greatly to my surprise, it passes them both:
On arrival in the United Kingdom, trained Officers will scan two fingerprints on an electronic fingerprint reader at border control. In the majority of cases we will use the right hand thumb and first finger.
Passengers will have to provide their fingerprints each time they travel to the United Kingdom [...] Fingerprints will be held for a maximum of 48 hours, after which time they will be destroyed.
So let's hear it for the UK Border Agency, an organisation that may actually be doing its job without unnecessary Stalinism. US DHS, take note.