1. Can I take my own food/drink
Yes, although nothing hot

2. Can I take an mp3 player
Yes, but you can't use it until pilot gives the OK for electronic devices

3. Can I use a laptop to watch my own DVD
Yes, but as above for when you can't use it

4. Can I take photos during the flight
Check with cabin crew, but pretty sure it's OK. Again, not until after you get the OK for electronic devices

5. I take prescription medication, do I need to have authority from my doctor to take these in carry-on (thyroxine)
No.

Other stuff:
DO get there early
DO get on the plane fairly early to get settled (and claim some overhead baggage space, it's limited)
DON'T expect anyone to help you (they might, but don't count on it)
DON'T take ridiculous amounts of carry-on. It's 7kg per person, in a bag that fits in those little frame things.
Thanks for the response.
When you said "No" about my medication question I just want to make sure that's "No you don't need authority from the doctor and yes you can take it on"
I'm planning to take my laptop and camera bag as carry-on (I have an SLR so can't fit the 2 in one bag). I (my uncle) has paid for me to have checked in baggage.

With regard to the medication, Would it be a good idea to take a copy of my prescription with me because I don't plan on taking the tablets in the box they come in, but rather will have them in pill boxes.