When is the best time to book a flight to Phuket or Bangkok?

Hi! Im basically looking to book flights from London for Christmas to visit my boyfriend in Bangkok or Phuket! Planning on surprising him as he is travelling at the moment, so for new years i've heard we will be hitting the full moon party in Koh Phangan, however would like a couple of weeks to travel around a few places beforehand! As i'm a university student i wanted to know the cheapest way i could do this and whether it be wise of me to wait until the very last minute to book, or if i should do it fairly soon.....also, one last thing! If any of you have been around Thailand, which place would you recommend on me staying around out of the two, i would like a nice, and fairly relaxing holiday with a beautiful beach nearby, any advice would be much appreciated! Websites with cheap fares, hotels etc etc, thanks!x