GPS does not use mobile data.
Live traffic updates do, and back in the day you could use it to assist in getting a faster initial satellite lock, but your in-car satnav does not require connectivity at all other than for routing alerts as you say. I have – rarely – hit issues with Google Maps dropping out if I’m in the middle of nowhere but always it resolves itself pretty quickly.
Call, me Mr Prepared if you like
Mr Paranoid more like. Still, it’s your money. Here:
https://www.moneysavingexpert.com/cheap-mobile-finder/sim-only/
Lebara, reselling Voda, £3/month overall with 36GB of data for the year, month-to-month rolling contract.