>>> No point in keeping fuel receipts
Agreed. Even if you had receipts, there would be no way of knowing whether the fuel was used for work travel or for getting to a naughty weekend in Brighton. The whole point of the fixed rate allowance scheme anyway is that it does away with the need to claim individual expenses. (e.g. you can't claim for the costs of servicing your vehicle, or for additional wear and tear, on it because such costs are all included within the flat rate payments).
As others have indicated, you can't claim mileage from your home to the Amazon depot at the start of each shift
https://www.gov.uk/hmrc-internal-manuals/employment-income-manual/eim32163 but you most definitely can to get home from your last drop.