My understanding is that if you come before the beginning of the UK tax year (some point in early april), you will pay tax on all income earned in tax year 12-13 but as it will only be a month or so's worth it is not significant. If you come after beginning of the tax year, you will pay full UK tax on all monies earned here in tax year 2013. You still have to pay NI contributions for 1 year after you arrive and then you can opt out but beware, this will affect your UK pension at a later date.

But as with all these things, better to take some advice.