Regarding notice, you need to call Motability and discuss it with them, you can pretty much end the lease whenever you want (there may be a financial penalty if ending early).
If you have got to the end of your 3 year contract then you can end at any time and you will get the GCB once the car has been inspected (usually takes a few days).
I did this August last year and because I’d been a named driver on another policy (even though I had had a claim with RSAM so zero no claims bonus) I still managed fully comp protected for £440 for a 2L car so very reasonable.