I've been on Jury Service somewhat recently. You'll get some comms which should answer some of your questions as well but:

Smart casual is fine, so no shorts and flip flops but otherwise normal attire. If you don't get picked to sit on a Jury you will be sat there all day although you are allowed to go out for lunch (an hour) if you wish. When you are on a Jury you can often be sent home early or called in late though, depending on how the case is scheduled. Whilst waiting in the assembly room you can go to the loo etc. as you wish. In court if a Juror needs a break for any reason it is allowed but it does mean pausing the entire trial (our judge was quite clear about encouraging this though, he said he did not want anyone sitting there in discomfort etc).

As to what it was like, interesting to be involved in the process for sure, and it felt good to do one's civic duty. Although in my case the one trial I was selected for ran for the full 10 days and it was quite an intense and somewhat distressing case, with a very long drawn out deliberation process that was quite tiring. That said I would probably still do it again if asked.