Visa renewal

Good day to all, I'm bit confused with the regards and would like to ask your expertise.

My contract will finish on 15th of November which I worked for 8 years. I have no intention of renewing my contract and would like to find a new one. Regardless, my manager asked me to email him my resignation.

Question is... should I write a resignation letter? Knowing my contract will expire on the above date.
2nd Q. Should I apply mobility before or after the visa cancellation?

Hoping for clarification on the above. Thank you for your time in reading it.

Your visa needs to be valid for 3 months at least in order for mobility to be applied.

Email is fine. Letter is needed if they are not accepting resignation or giving mobility. In which case you send the letter via registered mail and retain receipt. And then go to lmra to apply for transfer without employer consent.

Thank you.