I don't know anything about the tests themselves, but if you have applied for a post with the BAA and they have invited you to an assessment test, they must have seen in your application something that they are looking for. If they hadn't, they wouldn't bother wasting their time in inviting you along.
You have just as much chance as anyone else going to the test of getting through it. Think positive. You've got through the application process, you can get through the test.
Go along and do the test. Even if you fail it, you will know what to expect next time you apply for a job.
Good luck, and have self belief!