You have to have a really genuine reason, like major illness, or being dead!
I served on a jury many years ago (when Adam was a lad!) and it was a really interesting case of GBH and associated offences. I got two weeks off work, paid expenses for travel costs, which in my case were a lot less than the allowance paid, and a food allowance as well. I don't know if all that still applies today, but even so I think you should do it and take an interest in all that happens, and in the court as well. They are often fascinating places. It really is worthwhile, and you feel really good afterwards because you have done something to help a victim (usually).