Hello. I'm an accountant but not a personal tax expert, however I'd be surprised if Apple deduct any tax. I don't think it matters where the apps are sold geographically - if you conduct your business from the UK then you will be liable to UK tax.
If it were me, I'd initially imagine the following will apply, although I would contact Her Majestys Revenue and Customs (HMRC) to confirm:
If you are developing as a sole trader (you have not set up a company and are developing through that) then you will need to register as self employed with HMRC. If you have set up a company then you will need to complete a corporation tax return. I will assume for the moment that you are developing as a sole trader but let me know if you're not.
You will have to draw up a set of accounts that record the income that you have collected and any expense you have incurred. This could be expenditure such as software, books, stationery, a % of your internet if you work from home (although I think this would have to be agreed with HMRC prior to inclusion); essentially any cost you have incurred in the course of creating your app. You can draw these up separately or you can simply fill it out on the HMRC website.
This will net down to a profit or a loss depending on how much cost you have incurred. If you have made a profit then you will be liable to income tax on this profit at 20% or 40% depending on how much other income you have earned in the year.
If you have made a loss then you have a few options - I won't go into them to keep things simple at the moment on the assumption that you haven't spent over £6,500 on making your app!
You will also be liable for class 2 and class 4 national insurance. Class 2 is currently around £2.40 a week. Class 4 is 8% of your profit above £5,715, dropping to 1% above £43,875. You will need to register for class 2 NIC and pay this throughout the year. I believe class 4 is paid at the end of the year as there is a place on the tax return to declare it.
Also, if you have earned the income before April 6 2010 then you should be filling out a tax return for the tax year to April 2010, which means the tax return needs to be filed and the tax paid by 31 January 2011.
I'm not a tax expert at all, so if you want proper sound advice I'd talk to someone who specialises in personal tax - the above is the basics. There are various other things to consider too, such as deciding your year end and the effect this has on the overlap period you are taxed on, options for early year loss reliefs etc. To get this right I'd speak to HMRC at the very least, or a personal tax expert.
and i think they were getting cross with my stupidity 
