I do my bike insurance on-line, if you want a paper certificate, they will send you one, or just print it from the email they send you.
My worry is that, as good as it sounds, these things just don't always work, staff errors(computer errors) and such like can be a right pain in the arris, and if it does go wrong and you get pulled over, it can be extremely difficult to not have your bike/car impounded, not to mention trying to tax a vehicle if the insurance company haven't put the bike/car on the MIB!
And will the same apply if you still tax at a post office? because the paper police at the counters are a nightmare at the best of times.
As for the SORN, what a waste of money and time, it was when it was introduced and it still is! Why the hell do we need to tell the government that we are not using the bike? With all the cameras and the like, if you do ride it you'll get caught anyway!
Put a few pence on a litre of petrol and scrap road tax, the more petrol you use, the more tax you pay, why should I pay the same tax for riding my bike a few weekends a year as someone who rides up and down the country everyday?
Insurance needs a HUGE review too, pay one premium based on age and experience and you should be insured to drive anything, no matter how many bikes/cars you own, you can only drive one at a time anyway.