This is what you said below
Going by what you have said , You have a £1000 then you buy a £800 camera this means you cannot buy anything else which is more than the £200 you have left. Alternatively You start with £1000 in you bank, You spend £800 on the camera, £50 on a dinner then go home. The next day the payment for the meal has not yet gone through you check your balance quickly and See you have £200 great I'll spend £200 . When all the payments go through you now realize you are £50 in dept, in the old days the bank would now fine you for the unplanned over draft and you would now be £75 in dept. This has happened to me multiple times in the past so it does happen. Do the same scenario looking at the cash you have in your wallet, you would see straight away you don't have enough for a £200 purchase. Now if every thing you brought on you card was instantly taken from your bank account then your idea would work.
Beside all that why are you still arguing something that has nothing to do with the original question ?