£1200? Where does a 13 year old get money like that?
You are honestly mad to consider paying that for any sporting event, even F1 is far cheaper to go to. Even more so at your age.
You will find its pretty damn cheap to see GT/F3, I'd be surprised if you need more than £200 for the whole thing. It cost me £12 entry to see them at Oulton Park....
As for the event itself, yes its worth seeing it, especially if you are new to motorsport events. I haven't been to Brands, so I wouldn't know about the facilities, best spots, etc but I recommend you have a good look around during the events so you can see some of the paddock and watch the racing in different spots. This helps you learn where you prefer to watch for future events and personally I feel I've made the most of the experience.
I definitely suggest you visit the paddock at some point, being up close to the cars and mechanics as they work on the cars is half of the experience! The general bustle of the paddock is a great atmosphere too.
The GT run an hour-long endurance-style race whereas the F3 and F2 will run shorter sprint races. There should be support races included as well such as VW Cup, Caterham Championship or the Lotus Elise Trophy. Its all good fun and a good introduction into the many flavours of motor-racing (as well as a lesson in the various joys of engine sounds!). Its not worth £1200 though, but it is worth attending.
Personally I'd suggest trying to go to the BTCC races at Brands for a much bigger event and even more "bang for your buck".
If I was spending £1200 though, I would be going to the British Grand Prix, BTCC AND the GT/F3/F2. In fact, I am doing all 3 this year

. I'm certainly not going to be spending that silly amount of money though! Why don't you find some way to travel to and from the event in one day rather than spending all this money on accomodation?