iPad mail storage

If you are content that all email messages that you need to keep are visible to you via BTs webmail - you can completely erase all currently downloaded email from your iPad, for the account, by deleting the associated account from your iPad.

• Delete account:

Settings > Mail > Accounts > [Your Mail Account] > Delete Account

After deleting the account, perform a forced restart before re-adding the email account to your iPad.

• Force restart the iPad:

Force restart iPad - Apple Support

• Add Mail Account:

Settings > Mail > Add Account

After re-adding the account to your iPad, mail messages that are saved on BTs Mail server will re-populate on your iPad.

Both IMAP and POP3 mailboxes will store downloaded email messages on your iPad. The older POP3 mail protocol could be configured to either:

• Download and leave a copy on the Server
• Download and delete from the Server

If mail was deleted from the server when downloaded, the only copy of the mail message will reside on the device until manually deleted; after deletion, the associated message(s) cannot be recovered or re-downloaded.

Unless you only have basic BT Mail, you will be able to access your BT Mail using either IMAP or POP3; when adding a mail account to your iPad using the Add Mail wizard, the default configuration for many Mail Providers (including BT Mail) will be IMAP; if using POP3/POP3S, the account must be manually configured - providing all the server/port connection details for the POP and SMTP server settings. From your description, the latter is perhaps unlikely.

If you have many years of email stored on your iPad, you may indeed accumulate several Gigabytes of data. While mail messages themselves do not consume much space, attachments (such as files and photos) may account for a considerable proportion of saved data.

IMAP does not mean that the mails are only stored on the server it just mean that it is also stored on the server

What is IMAP and POP3? – Support | one.com