As long as you have one ordered online I believe there is a grace period for it to arrive.
You have a grace period if you've ordered it online. Everything is stored in a database anyway so the police would be able to see you've paid for it.
you would imagine this is the case- I had to fight a fixed penalty for non-display when I had ordered my tax disc online and it never arrived in time, I had to provide the reciepts for payments and confirmation emails etc from the dvla - rather than the police actually bothering to check anything
(that was with the out of date one still on the car btw)
I got mine today and as its for a bike I'm going to photocopy it at work and display that one. The real one will be in my wallet.