They calculate the tax credits on last year's figures, because the figures are verifiable, and for most people it won't be dramatically different this year. And then they should make adjustments later if this year's figures are different. If your circumstances have changed dramatically - either income or childcare costs - as in your case, then they should work out new figures based on the current status.
As long as you work 16 hours a week or more then you may well be able to get some help with childcare (depending upon your income of course).
If you want to go back, but not necessarily full-time, then it is probably worth trying to get your hours reduced to 16 if you can, as it's often easier to get a full time job reduced to part time - and retain your exisiting pay rates -than it is to find a suitable part time job which pays the same.
Although surprisingly it may not be worth you working much more than that unless you are fairly well paid, relative to your additional childcare costs.
The childcare tax credit is 70% of childcare costs (up to a maximum of £135 per week) - but they reduce this at a rate of 37p in the £1 for every extra pound you earn. Bearing in mind that on that £1 you'll also probably be paying tax at 22% and National Insurance at 11%, that's effectively a whopping 70% tax rate, and you'll still have to pay 30% of your additional childcare costs out of what's left.
Let's assume that we are starting from a position of you working 16 hours a week (the minimum to qualify for the childcare element of the tax credit) which will also allow you to benefit from your personal tax allowance, but are not at this level at the maximum £135 per week childcare spend....
Say your childcare cost is £4 an hour, and you are paid £5 an hour. Working one additonal hour will increase your income by £5 and your childcare costs by £4.
However the childcare element of child tax credit would increase by £4 x 70% = £2.80
But because your income had increased it would be taken away again at a rate of 37% of £5 - i.e. £1.85
You'd also be paying additional tax @ 22% and NI @ 11% - i.e. a total of £1.65.
Total:
+£5 (pay)
+£2.80 (CTC)
-£4 (childcare)
-£1.85 (CTC)
-£1.65 (tax)
Overall result: better off by 30p !
Try the calculator based on your projected position with childcare, and ignore the "use last years figures" bit to get a better idea for your circumstances.
Try the scenario of going back on your current hours/salaray, and paying childcare for that time, and compare it with the incomes/cost if you could go back for 16 hours (with salary pro-rata). You might find that there is little difference. On the example above, going from 16 hours a week to a full time 37.5 would leave you just £6.45 a week better off (some of which would probably be swallowed by extra transport costs etc in order to work the extra time).