I personally fail to understand why they don't setup some sort of computer service for the post office to use, similar to the one that the police use to them whether a car has valid insurance, mot certificate etc...
It would make it much simpler for the post office to sort things out, instead of them taking forever to read through the docs and causing problems for people who may have lost their documents, they can simply then type in the reg and they will know whether or not they can issue tax pretty much straight away (plus they could also be prompted if the car has been registered stolen and/or write off).
To be honest, the government say that the reason you need an insurance certificate is to ensure that your car is insured, however in theory I could go in and get tax with a certificate of insurance that was issued 9 months ago but that has no longer been valid for the past 7 months due to non payment and/or cancellation.
So really it does not help keep un-insured vehicles off the road much.
Just my 2 pence.