There could be many reasons (commercial, legal) for organisations keeping personal data for a certain period of time. Unfortunately, the Data Protection Act does not specify the length of time it can be kept for as it could be different in different situations:
https://ico.org.uk/for-organisations/guide-to-data-p...
I am not sure if it is applicable to personal data, but the European Union's Data Retention Directive also specifies that all telecommunications data in the UK is kept for a minimum of one year and a maximum of two years.