Does data rollover?

Patc66  In short, no. A plan lasts for one month and when it's over, it really is over.

This is the thing about not being tied into any lengthy contract.

The only other possibility into purchase a data discount plan if you regularly do not use much data - https://smarty.co.uk/all-plans#data-discount. This gives you money back towards your next plan to cover the value of unused data. You need to calculate if you would be better off as the plan is dearer at the outset.