Hang on to your P45/60.
Don't worry about the gratuity, it's not included in any of the calculations.
Make sure your pension has the correct tax code ( taxed at 40%) and your new main income tax code is also correct. ( this can take ages to sort.)
Be aware that if your gross earnings top £100k for the year your tax free allowance is gradually withdrawn. This means between £100k and ~ £118k you are taxed at something like 60%!
Either way, my advice would be to just do a selfie tax return but keep a slush fund hady to pay the inevitable bill.