A previously booked holiday is a legitimate excuse for deferment.
If you ring them up, you should be able to agree what date you do it. I've just changed the date for my 3rd lot of service.
To be honest be prepared for two weeks of boredom, there's an awful lot of waiting around, interspersed with a few moments of interest. By the time you've trooped in & out, had a few recesses, decided its too late in the day to start another case, had a long lunch break etc I reckon you probably only spend something like 10 hours in a week actually in a court room listening to a case.
Whilst I appreciate its your duty to do this sort of thing, as a self employed person the £64/day to cover loss of earnings is an absolute insult. Reduced of course to £32 if you get sent home early.