Technophobe

Hi, I'm a new member, and I'm changing my XF Portfolio (MY2014) to an XJ Portfolio (MY2016) this month. I've got a 'Three' SIM card in my phone, but need to purchase a 'data only SIM' for the on-board WiFi. I spoke to 'Three' but the sales guy said that sometimes their SIMs hadn't worked in some cars (but wasn't more specific). Hence the question: Has anyone got any good/bad experience with Data only SIMs? Which networks are good or to be avoided? Any help gratefully received.