Do I need to tell ucas I am resitting an exam

I wrote exams in 2024 and have been on a gap year(s) since then. I am currently reapplying to uni and was considering just doing a foundation year, but decided to resit for chances to get into good unis. I got a reference from my old school, but since it's been so long, they couldn't give me a predicted grade. How do I get a predicted grade, or do I just leave it out of my application completely?

No, you have to include the fact that you're resitting into UCAS, else you're technically committing application fraud, which is a very nice way to shoot yourself in the foot
The predicted grade is the responsibility of your reference. It's relatively common and OK to do resits and not have predicted grades for them, unis are used to this sort of scenario

Hi, I'm planning to resit my a levels even though I already have an unconditional deferred offer from this summer (just to keep myself updated with content + try for a better grade during my gap year) and am wondering if I need to make any changes to my UCAS about this? I don't plan on reapplying anywhere but I don't know if my grades will automatically update on my official records (if its related to my UCI / ULN numbers)?