Wouldn't that in itself fall afoul of Data Protection laws and therefore make them liable?
You could always pay the £10 after you've closed the account and see for yourself

From their website :
7. Access to information
You have the right under the Data Protection Act 1998 to access the information we hold on you. You may submit a request to us in writing accompanied with the applicable £10 fee.
The Data Protection Act however does not enforce them to remove all traces of your details, nor do they promise to do that. It simply states that they will hold it securely and not keep unnecessary details. However this sometimes goes a bit wrong too :
http://www.eva-scotland.org/forum/viewtopic.php?f=8&t=192
Most companies decide their own data retention policies and as long as they are reasonable, have no problems with the ICO. Infact the ICO guidelines state :
You may not need to delete all personal data when the relationship ends. You may need to keep some information so that you can confirm that the relationship existed – and that it has ended – as well as some of its details.
More info here :
https://ico.org.uk/for-organisations/guide-to-data-protection/principle-5-retention/
The obvious processes are followed to conform to PCIDSS - payment card details are not stored. Payment history however, is.
The truth is - once you are a customer, you're forever in the system.