Erm, you certainly are (at least in the UK). If you don't reply to the letter you face a minimum £1000 fine and possible jail. I know this as I did Jury Service a few months ago. If it's your first time on jury service then you are allowed to defer once for up to a year if you have a really, really pressing reason why. However, you do have to serve. The only way out is having a criminal record or being a judge.
Anyway, Jury Service can be really fascinating (and really boring). I'm certainly glad I did it when I was called as it was really interesting (and rather comic). Well worth doing if you get the chance.