This can and certainly does happen Andy, and any controller will tell you that when storm cells are around, vertical separation is a beautiful thing..........
Vertical separation is indeed a beautiful thing. However on Sunday in the LTMA when the weather suddenly hit, there were aircraft all over the place, some wandering through 3 or four different sectors that they would normally never be near. Level separation was not an option!!
Clouds, however active, are softer than aluminium. Always ask first, I will MOR a pilot who puts his and other aircraft into danger because he turns before asking. I will remind a pilot of his obligation to request a trun if his actions do not happen to put himself in harms way on that day.
It will be a very rare and extremely rare happening if you are refused the turn you want - if you are, then it is because you will endanger your, and other crews, aircraft.
I know the ride isn't pleasant (I flew professionally before this career), but it is not safe to turn without permission!!
I am campaigning to get an educational DVD produced by NATS that will illustrate the effect weather can have - it will include actual radar replays with R/T and phone calls. It is initially intended for Tower controllers as part of TRUCE (cos we don't like being badgered every 5 mins to cancel MDIs etc when we are hanging on for dear life), but I have suggested it gets sent to the airlines as well.
Unfortunately the safety manager who was supposed to produce this DVD after my suggestion several months ago (about 12) has been very slow and in fact very negligent in doing so. He was given a sharp reminder by his peers very recently.