Remember that the actual sentence served is (for short sentences) only half of that imposed by the court. So if you get sentenced to 3 months, you'll only serve about 6� weeks. Any days where you spent some time at a police station, or in initial court hearings, count as 'police days' and are deducted from your sentence. Additionally, if your official release day is a Saturday or Sunday, you'll be released on the Friday morning. (So your actual time in prison, for a 3 month sentence, could be less than 6 weeks. You might even be offered a tagging scheme and get out even earlier).
In reply to your earlier question, someone unhelpfully wrote 'pack a toothbrush'. What you really need to pack is a radio, CD or MP3 player, CDs (if relevant), spare batteries, some books, writing paper, pens, envelopes, postage stamps and your address book. Put them in a bag and (most important) keep the bag with you. (If you hand the bag to a friend or relative, they won't be able to get it to you after you're 'sent down'. If you're not allowed to take the bag into the dock, hand it to a court official).
It would also be sensible to make sure that you've got some money in your pocket when you're sentenced. (Perhaps �50+?). Prisoners buy 'luxuries' (like tobacco or biscuits) from the money they earn from work or through attending education courses. However, it's unlikely that you'd be be there long enough to be allocated a job or a place on a course, so you'd only receive the 'unemployed' rate of pay. Any money in your pocket on arrival would be taken from you, but credited to your spending account, so that you could buy a few luxuries with it.
Chris