Sorry I wasn't here earlier, there are a LOT of things to pick up on here....
Just to confirm, child care services are exempt from VAT. This means that you do not charge VAT on your services, but you cannot recover VAT on your expenses. The fact that you are not a company or that your turnover is low has nothing to do with it - the situation is exactly the same for the biggest nursery chain in the country.
NCMA don't provide individual tax advice, you could join the Childminding Forum as there is a helpful section there, or look for an accountant - it could be cheaper than you think (certainly less than £300 a year, possibly only 1/3 of that).
Major purchases (things that last for more than a year) are claimed as Annual Investment Allowance - if you only use the item for your business you claim the whole of the cost, if you use it 25% of the time for non-business stuff you claim 75%. This reduces the amount you are taxed on by the amount you claim, and therefore you pay 20% of that amount less tax (assuming you pay basic rate tax).
As well as tax you pay Class 4 National Insurance at 9% BUT you only start paying that and income tax once you earn over the thresholds which are slightly different, about £7,800 for 2012/13. So to save the right amount, work out your profit each month (don't forget the 10% wear and tear allowance), take off £660 and save 29% of what's left. You won't have to pay a penny until January 2014 (yes, 2014) so you should have built up a fair balance by then.
If you are earning in another job you could get a tax refund for your expenses this year, but if you can afford to wait it will be better as you will then save the Class 4 NI as well.
You didn't need to register as self employed until you started trading (which is usually taken to be when you actually start minding, or take a deposit), but you have now - no matter. You can submit a nil self employed tax return for this year (no income, no expenses, no nothing) and put all the expenses in next year (this is a special situation for dealing with what is called 'pre-trading expenses', you can't normally do this).
WOAH just read the higher rate taxpayer bit, you presumably won't be in 2012/13 on just 4 months of your current earnings so better to start trading NOW to take as much expenses as you can in this year. It might be an idea to do some weekend babysitting for money before the end of March to make sure you can show your business has commenced. With higher rate tax at stake, you really should get a decent accountant - if you paid £400 for that laptop, I have just saved you £44 so decent advice should pay for itself many times over.