Sharky,
I'm not eligible this year so all I can comment on was last year.
There were 12 applicants to the final stage of which 6 were required. I gather it is the same situation this year.
If you are well prepared then you have every chance of progressing and I saw recently the youngest and oldest candidates last year were 18 and 31 respectively. The first day is the standard batch of OAT tests, with some slight variations and a much higher minimum standard. You will know by the end of the day if you were successful enough to progress to the next day.
That in itself could be worth the £200 - to have reached day two is a pretty good sign and confidence booster, although to be knocked out at that stage would no doubt feel like a very poor investment of £200. The second day was good fun and the interview process nothing to be feared.
The best advice is to prepare. Think about your strengths and weakenesses, about what airlines are looking for, your involvement or knowledge of aviation and the industry and of course do consider what experiences in your life you can use to demonstrate your abilities. Get comfortable with making your points clearly and positively and you will walk in to the interviews and other aspects with more confidence.
Despite all that has been said here, it is a golden opportunity. Given the costs of aviation training, £200 spent now is a drop in the pond and given the choice between 3 hours flying time or this selection process I would recommend the later. At least if it doesn't work out, you'll know where you stand.