Lost BRP Waiting for Replacement

[pokekap]

Hi everyone,
I lost my BRP when I was in the states a few months ago.

My timeline:
Applied for Replacement BRP (after getting back to the UK): 1st July
Biometrics taken: 21st July

I am still waiting for my new BRP.

My visa expires in September second week so I have to apply for an extension before that.
I just wanted to know if I should wait for the new card or if I should just apply for the extension right away.

I am a Tier 1 Graduate Entrepreneur.

Thanks in advance.