I've just started at the CATC in Hurn (hence the handle) in Bournemouth. It took two years in total for me to gett all the selection crap out of the way and to get into the college itself. It is definitely worth it though, they treat you like royalty once you get there. As for the cubes, there is no way to practice, you just have to get the nack of looking at which ones definitely dont fit, and work from there. If you write to NATS they will tell you what you failed on. my first time round i failed on the test with dots and arrows...
Keep trying, there are people starting with me who didnt get in until their third attempt, nothing wrong with at all.
It really is worth it.