I got summonsed for jury service a couple of months ago. Work asked me to request a deferral as we are very busy at the moment. The deferral was successful and I got another date through (July, I think).
I'm actually looking forward to it. Think about it, two weeks off work, almost certainly fully paid (mine is, and most companies will top up your payments up to your salary). And it's bound to be pretty interesting, as it's the more serious cases that need a jury.
Edited to say, I would guess that a pre-booked holiday is sufficient reason for a deferral.