It never ceases to amaze me that people can ask for a 'crash' course in driving, completely oblivious to the irony of such a request....
Why do you need a 'crash' course? Start now and have regular lessons if you can.
Obviously if you've already booked a test you need to point it out when you book the course. You shouldn't be double charged.
The main problem with booking the test yourself is that you might find the instructor is not available when you want him/her. But you can reschedule a test upto 3 working days before the test date.
If you do take, for example, a week's intensive course what should happen is that you get a couple of days tuition, then decide whether to go ahead with the test or reschedule it. The problem is that if you decide at the last minute you are not going to be ready you might have to wait another 6 weeks before you get another go.
It's really better to start lessons sooner rather than later, then if after a few weeks it's not going as well as you'd hoped you have more flexibility to give yourself more time/practice/lessons. Going for a day a week or a couple of evenings a week is the most efficient way to learn. Most people don't appreciate how hard it is to drive all week under pressure.