What are my options for extending my Thai visa if I cannot return to Thailand until 2026 due to health issues?

Sadly no, if you don't make it back before your current extension expires it runs out and you lose it.

You could buy a new 90 day Non-O visa from the Thai consulate in London online before you come back via the eVisa web portal

Arrive, get a 90-day entry stamp, and when you have 30 days or less left on that apply for a new yearly extension at your immigration office like normal.

OR

You could also come in free stamp entry on a 60-day visa exempt entry, then first apply for the in-country 90-day Non-o visa at your immigration office (if they are one of the offices that issue in country visas) and after you got that, apply for the year extension.

Of the two choices coming in with the VISA already in possession is the easier of the two as you then only need to get a year extension when it runs down