I would have liked to be on a jury, but I wasn't picked either time! AFAIR there were about 200 people called for whatever trials were starting that week, so maybe 36 people are picked. Anyone with a reasonable reason, pressure of work, or in one case someone who had experianced that particular crime in his family, were excused from a particular jury. They describe in broad outline what the case is and how long it is expected to last, so that if it is not possible for you to be there for the expected length then you can say so at the beginning.
In my experiance you spend the morning waiting (they take a roll call) and then get the cases sorted out before lunch, then (I think) you are dismissed for the day and told whether or not you have to come back the next day. In my case, I went off to work in the afternoon as it wasn't that far away from the Criminal Courts of Justice.
ETA, like I said it wasn't far from work, so it wasn't any extra travelling cost (in fact it was cheaper to get there than to get to work) and I just brought my usual packed lunch, so it isn't something that cost me money.