Take your time with the job and pace yourself. The money will always be there, but your licence might not if you try to force it.
When you're tired, it's easy to make mistakes. You stop noticing the 20mph speed limits, you accept that dreaded "last job" that takes you to the other side of London, or you find yourself tempted to check your mobile phone.
None of those are worth it. One bad decision can easily cost you the next day's work... or worse, your licence.
If you can, buy your taxi as soon as you feel comfortable doing so. I really can't stress this enough.
I made the mistake of waiting seven years because I was worried about the costs if something went wrong. You hear plenty of horror stories, but you'd have to be very unlucky. Looking back, I wish I'd taken the plunge much sooner.