The GAPAN tests are worth doing, as has been mentioned, even just for some assurance that you are capable of making the grade. Scroggs is correct in saying that the tests are designed to see if you are capable of handling information required to pass sim and line flying checks in a busy environment such as commercial flying on big jets. Failing the GAPAN tests doesnt mean that you cannot make it onto the flight deck, but remember that as there is continual assessmnet throughout your flying career, you will need to keep up the level of work required to pass your CPL all the way through your career. So if you are scared of being assessed, then you are not going to like commercial flying, as there is always someone looking over your shoulder.
I have done the tests and found them ok, just take your time and make sure that you understand what it is that they are asking you to do. You dont need to have any flying knowledge.
150 quid is a drop in the ocean compared to the mountain of money that you are going to spend on training. Better to have an idea now than at the end.
The GAPAN interview thrown in is also valuable, as it does not only take the brains to make a pilot but you have to have a personality. Remember someone is going to be sat next to you for a long time and will want to get along with you, as well the rest of the crew.
Take the plunge and do it.