After completing a job, with no WP, can I stay for 28 days ?

Currently I am working in the UK
as a work permit holder. I am getting another job
offer which is sort of a contract (but not
self-employment. I will be employee of an umbrella
company) for 6 months. Now my queries are:

1> If my prospective employer files my new WP and I
receive the WP approval from the Home Office then can
I start my new job immediately after that or do I need
to wait till I fill up the FLR(IED) form and get the
endorsement on the passport?

2> If I take this new opportunity then at the end of 6
months, when my contract will be over, there could be 2 scenarios :

A> At the end of my contract I DO HAVE a leave to remain in
the UK (in case I was granted the "leave to remain" for more than 6 months ... say 9 months while starting this new contract) for more than 28 days. In this case also do I have only 28 days to search for another job ? or could I stay longer in the UK for searching another job ?

B> At the end of my job I DO NOT HAVE a leave to
remain in the UK (In case I was grated the "leave to remain" for exactly 6 months while starting this new contract). In this case also am I allowed to
stay for upto 28 days to search for another job ?(considering I don't have a leave to remain in the UK for these 28 days)

Thanks in advance!

see details on the www.workingintheuk.gov.uk website. All answeres are there and on this forum - you just gotta do some legwork for yourself