1) Whenever you can get it and whenever the hospital will allow you to get experience in clinical areas. In some (most?) hospitals this is when you're 17 or over.
2) What's the question?
I would try and see if you can arrange it for during the holidays. If you can't get time off for it, then that's just unfortunately bad luck and yeah you can always try the 16+ one. See if there are further details about it on the hospital website or perhaps there is a work experience co-ordinator you can contact.
3) Yes, and I got to do this (ENT surgery if you're interested). At the time I vaguely thought I was entering the inner sanctum of the hospital because literally, who's cooler than the surgeons right?!
I then spent many a med school surgical placement craning my neck around the surgical consultant's back and wishing they would hurry up and close so I could rest my aching feet. Basically, it's not as interesting as you think so if you don't get it, you're not really missing out and you'll get more than enough theatre time in med school if you want it.
4) Yes absolutely. Pump your contacts mercilessly for tips, further contacts, a foot in the work experience door, etc. It's a shame that in this day and age a significant part of getting work experience is about having the right contacts, but that's just unfortunately the way it is.
5) Write letters or emails to the practice manager. If you're not getting replies, chase them up. Don't be surprised if they simply say no, GP work experience is hard to arrange.