To maintain your personal security then full deletion of a redundant onetel email account is a good decision.
The Data Protection Team will delete an email account for you under the General Data Protection Regulation (GDPR) Right to Erasure. Here's the TalkTalk advice article related to the GDPR. Scroll down and select the section "Your right to be forgotten also known as Right to Erasure" for making a postal request for your email account and any contents to be authorised for deletion by the Data Protection team.
As well as your identification they'll need proof that you're the registered user of the email address including:
Account verification is managed by the Data Protection team before email account deletion is authorised as deletion of personal data is not reversible.
Reports say that the process is handled quickly but do please update us on the outcome as it's good to get feedback.