Once arrested for a recordable offence (legal wording for most criminal and some driving offences) your finger prints, photograph and DNA are collected. At this time you will also have a file created on the Police National Computer that is specific to you and records the results and any offences for which you have been arrested. If you are later cautioned or convicted you will also get a CRO (number on the criminal record) which is also stored on the PNC. Your PNC file remains on the system until you would turn 100 years old (it being felt that there aren't many 100 year old criminals).
The DNA and fingerprints remain on the file for as long as it exists, regardless of the outcome of the incident for which you are arrested. As stated the actual sample of your DNA is disposed of however the data obtained is retained on file somewhere electronically.