It doesn't. It's not eligible for the piggybank. If you've only got unlimited plan(s) on your account then you don't save any data into your bank. If you upgrade to an unlimited plan then you stop banking any data on that plan from that date. If you've got other plans on the account that aren't unlimited they'll still be able to use it, and even if you don't any data you've already banked is available up until the expiry date (currently 12 months), so if you downgrade from unlimited later it's still available.
https://www.sky.com/help/articles/sky-mobile-sky-piggybank
