Charged a whole year supply for old address almost 2 years after I moved out

Just to note that having a Tenancy at one property doesn't automatically mean you are not liable for the bills at another property.

For example, a person could have/rent 2 properties and then sub-let one as 'all inclusive' of bills.
I'm not suggesting for a moment that you have been doing that, it's just one example.*

Somebody obviously believes that you are still liable for the bill at the old property for some reason, otherwise there would be no point in them taking you to court.
You don’t start a court action unless you are pretty sure (rightly or wrongly) that you are going to be sucessful.

We don't know what that reason may be

However they'll now have to convince the court that whatever they think makes you liable to pay the bill then that reason is correct in law.

*One slightly concerning thought/possibility does come to mind here, you say:

.... I gave final meter readings over the phone as I've always done when I moved and thought nothing else of it.

However you don't mention getting a final bill after you moved out and new tenants moved in.
You don’t mention getting any confirmation that the account had been closed.

Is it possible that the account in your name was never closed properly?, and that the new tenants never registered new utility accounts there? So the account remained still active in your name until March 2023? Maybe when those tenants left and someone else moved in and registered new utility accounts?
(Such things can and do happen in rental properties).

If that is what happened then that's an administrative error, either made by you for not closing your account properly, or made by the supplier not closing it after you told them to do so.

In which case the court would need to decide who’s error it was, and if in fact you were still liable even though you were no longer living there.

Please do let us know how you get on.