Timing of handing over notice period

[rickmanicar]

Hi,

I'm new to this forum but I'd like to ask for some advice.

I've recently received a new offer with another employer and have agreed on an start date in mid-October in order for me to re-locate in London.
Because I still have yet to resign with my current employer and work out the notice period of 3 weeks I've got with them. I'd like to get some advice on how the letter of resignation procedure and timing works. As I'll have to resign slightly before, to getting my application sent to the Home Office.

I know that while my application is being reviewed I should be covered by my new employer's visa process, I just want to know for how long does my current employer sponsorship covers between me working out the notice period and how long is the current curtailment of my visa while I await the response from the Home Office. Ideally, I'd like to understand the timeline in terms of how many days/weeks/months do I have before my current visa gets curtailed and finished while I work out the new visa process in the meantime.