Actual start date later than the Cos start date (1month)

[ayhanisik]

Hello, I hope someone could guide me on this:
I was on Tier 2 visa and for my new job, I applied for an extension which was approved.
I received my BRP last week.
I was supposed to start my job on the 4th of November, but HR tells me that the contract cannot come through before one month. Sometimes it even takes longer.
I haven't left the country yet, as I was waiting for my visa.
My question is, what is the allowed window between the start date stated on my CoS and my actual start date?
Another thing is: am I allowed to stay in the country before starting my job?
Thank you in advance for your help.