You have to become a non-resident. To qualify for this you have to be out of the country for a complete tax year (april to april), but you can claim non-residency prior to completing that year just by filling in a form from the www.hmrc.gov.uk website.

However, if you claim non-residency and then subsequently don't stay out of the UK for a complete tax year then you'll have to pay back any tax you owe.

The HMRC website contains lots of information for non-residents. You should definitely have a look.