What I'm trying to explain quite badly is that you don't. You pay tax on everything over £10,000 that you earn each year. They calculate your tax month on month rather than at the end of the year. This means that, each month, you effectively have £833 tax free allowance, which adds up to your total tax free allowance each year.
So, say, months 1-6, you earn £2000 each month, and pay tax on £1177 of that each month (2000-tax free allowance of 833). So far you have used up 6 x 833 = about £5000 of your total tax free allowance for the year.
Months 7-12 you are on mat leave and earn £600 a month. You still have approx £5000 tax free allowance for these six months, equating to approx £833 a month. Your earnings now fall below the threshold so you aren't taxed on them. In fact, as you are earning £233 a month BELOW the tax free allowance, this extra is offset against the tax you have already paid, and you get a small amount back.
Overall, in my example, over the whole year you have earned £15600, of which you have only paid tax on £5600, as the first £10,000 is your tax free allowance.