What happens to my data if I cancel JoltMile?

Nothing is deleted. Cancelling ends the subscription, not the account.

You keep every shift, expense and receipt, and the JSON backup, the CSV export and the Self Assessment export all keep working. Two things change. You go back to the free plan, which logs 12 new shifts a month, and the Self Assessment screen's figures and the filing help need a subscription again. Somebody who logged forty while subscribed and then cancelled keeps all forty and can read, export and back up every one.

That matters more than it might sound, because HMRC expects self-employed records to be kept for at least five years after the 31 January filing deadline. An app that took your records away when you stopped paying would leave you unable to meet an obligation you still have.

Cancel where you subscribed: in your iPhone's Settings under your name and Subscriptions, or in the Play Store under Payments and subscriptions. You keep access to the end of the period you have already paid for. Deleting your account is a separate, deliberate action - see account and billing.