The old $15 thermostat I picked up from autozone last year isn't opening all the way. Jaguar tells me they've had issues with using aftermarket thermostats and this could be the source of my no heat issue. So I replaced it with a genuine jaguar thermostat. Refilled with coolant and let the engine warm up. The temperature gauge kept climbing. Could I have gotten a bad thermostat? Or did I fudge something up? Didn't have this issue last year when I replaced it.

Fitted back to front?

Not entirely sure what you mean by that. However, after the car cooled down a little bit, I cracked open the radiator cap a little bit. I'm thinking maybe not all the air escaped out of the line and had some air bubbles or something, cause it's not overheating now. And it feels like my heat is working, though it's not as hot as I would expect it to be.