Switching Employers // Notice Period

Hello all,

I've been reading many posts on this forum regarding this topic but haven't been able to find a general consensus.

My situation: I am currently working for company A on a skilled worker visa. I have a new company, B, in the process of generating a CoS and applying to switch my visa for me. I have not yet seen a copy of the CoS so not sure what date they have/will put on it, but the start date agreed upon with company B is September 15th.

Question 1) I haven't given notice (4 weeks) with Company A yet and have found recommendations of both waiting until I get a decision email, and waiting until I receive the physical BRP to see the "valid from" date on there. My understanding is I can't work for my old employer past this "valid from" date on the BRP. Is this accurate? Or is the key date the one on the CoS? Is there a link anywhere on the gov website that definitively answers this?

Question 2) I am due to travel at the end of August for an immediate family member's wedding, which I cannot miss. Am I able to travel after receiving a decision & before receiving the BRP? Or do I need to have the BRP in hand before I travel?

Thanks for your help!

1. See also Skilled Worker Visa Update and Notice Period
2. Probably safer to wait for an BRP, although it may be digital.