Change of Employer in Tier II

Hi,

I am working with my existing employer who had sponsored my Tier II visa.
It is valid till Jul 2014.

But I am planning to change my Employer for better Salary and prospects.
They are A-rated sponsors and are ready to Sponsor my Tier II visa.

I've asked them to raise my CoS starting 1st of Nov.
And they would provide me with a CoS by 10th Oct for me to fill the Application.

From the existing timelines taken by UKBA. I doubt I'll get my BRP and visa done by 1st of Nov.

-Can I continue to work for my existing employer even after 1stNov till I am issued a new Biometric Residence Permit?

-Will my existing employer be notified by ukba that I have been allocated CoS by another company and
hence I am not allowed to work with them ?

-What if I am not allowed to work for my existing employer after 1 Nov and the New company rules doesn't allow me
join till I get the BRP with Approval letter?


(Is it not legal to employ people on the basis of an Approved CoS , Otherwise how could people apply for extension or change of employment,
If the new employer wants to see the BRP and approval letter during joining)

I understand it varies from one company to another but any views or comments on this would be highly appreciable.

Regards
JD