Exactly !!!! (And why it scares me)
Unlike fingerprints (which are also indexed) DNA samples do not (except in very rare cases) hold the entire sequence. Once a suspects DNA is taken it is reduced to an index (IIRC 10-point ? Although there are plans to change that), that index is ALL YOU HAVE.
So, posit: 2020, when the DNAD has 10,000,000 records. A crime is committed. Police find DNA, and enter into the DNAD ... and get 10 matches.
Bear in mind they haven't got the *original* DNA that goes behind these matches, so they can't home in on the individual which may (or may not) match their sample.
So they now have 10 potential suspects. They find and interview 4 immediately. 2 have since moved/changed names and take longer. One has died. One has emigrated. One was injured in an accident and can't walk. And one can't be found at all.
Now, any one of the 4 the police did find (and will presumably drag to court) will have a ready made "reasonable doubt" argument. By failing to eliminate the final match, unless the police have done some spectacularly good real police work, the defence can always say "but you never eliminated this person". Don't be surprised to hear government noises about how "beyond reasonable doubt" is a criminals charter, and really we should be satisifed with a "balance of probablilities" approach to jailing criminals.
What really scares me, is that it seems historically, when the UK faces obstacles to getting people locked up, in the shape of pesky civil liberties and protection from injustice, we seem to almost unerringly go the way of weakening the protections, rather than strengthening the investigation.
Of course once a UK court has played out the weakeness of holding so much DNA, it won't be long before foreign courts point to the UK as evidence of the weakeness of DNA testimony. Not something I can see the UK getting a "most popular member of the international law enforcement community" award for. I know from US friends in law enforcement, that some parts of the UK police are regarded as no better than the Marx brothers anyway.