You might have heard about the Warwick Insurance by now. My best advice is to take it out, definitely. It cost £143 this year, I think, which is such a bargain. I did not take it about because I plan to go to America over the summer, and the Warwick policy only covers you for the country you are studying in. However, in retrospect I wish I had taken it because it's a really comprehensive plan and covers your laptop, and the excess is only £25.
To put this into perspective, the cheapest I have found since that will cover me for a year in Canada and the USA costs £250, and it doesn't cover valuables, and only up to £200 for baggage with a £100 limit on any item, and the excess is £65. Not great.
So you should take out the Warwick policy immediately. If you want to travel over the summer, you can take out insurance for a few months quite cheaply. Backpackers insurance is easy to get hold of, but assumes that you don't carry loads of valuables with you.
So yes, DO IT. Another thing you will probably hear about is UHIP. This is the mandatory university health insurance. I think you have a form to fill in for it before you arrive, and then when you get to Kingston, you should go to the international centre (first floor of the JDUC) and sort it all out there. It costs $352 I think, and covers you for the eight months you spend here. Pretty simple!