Documents and Proof of address

So this is the quick and short.
I moved here in the last few days from South Africa with a brittish passport which I aquired via decent.
I'm currently staying with my Uncle and his family rent free for now untill I get my foot in the door.
I have applied for my National Insurance nr but I need to prove my address some how. What can be used to prove this?
I cant open a bank accout yet becuase I dont have a proof of address. How can I obtain one.
Before I accept a Job must I have a Uk bank account or can I declare the cash payment or does my payments become withheld untill I receive a Uk bank account.
Will a Job provide me with a proof of address to be able to open an account?
What needs to be done when it comes to National Health Service and what do I need to gain a NHS nr.
Are there any other documents I need to apply for?
Thanks for your time.

Ask your uncle to write a letter confirming you live there and provide a copy of his ID, coucil tax or utility.

For NHS, you need to register with a GP surgery. You will need the above letter and documents too.

Or ask your uncle to put your name on one of the utility bills for now so that you have something in your name.