Hi woodhouse
The simple way is to declare non residency when you move out of UK
A person who is non-UK resident: is taxed on (some) UK source income only. Such a person can bring non-UK income into the UK freely.
A person who is non-UK resident: is not subject to capital gains tax at all (even on disposals of UK assets). Such a person can bring non-UK gains into the UK freely
Simple explanation for residency and domicile status here, I just came across it, no affiliation with them

Your domicile and residency are two different things, you cannot change your domicile unless you break all ties with UK and gain domicile somewhere else. Normally the domicile is linked to where your father is born.
http://www.charlesrussell.co.uk/UserFiles/file/pdf/Private%20Client/Resi...
Secondly double tax treaty doesn’t mean that Qatar tax is 0% hence you do not pay any tax on income declare in self assessment- QDCL is quite right
Simply it means that you will not be taxed twice on your income. for example you will calculate tax on your foreign income using UK tax rates and deduct any tax (if there is any) you have already paid in other country, having double tax treaty and pay the rest to HMRC.
For example your foreign income is 100,000 pounds (earned in Qatar) for year 2012-2013 and you are classed as UK resident, you will pay tax & NI using HMRC tax rate of 20% & 40% , as you haven’t paid any tax in Qatar (0% income tax rate) there isn’t anything to deduct. Tax & NI works out to be around 35,000 (pounds)

Double tax treaty is mainly applied to UK companies having a branch here, who pay corporation tax at 10% here in Qatar and when they submit their tax return back in UK, they deduct this tax paid at10% from any tax liability due to HMRC

JJJZ: one can become non-resident any time in the year by filling in Form P85 (available from HMRC) - yes you are right if you leave the UK before 6th April and stay out for until 5th April the following year, and you are automatically classed as nonresident if your visit doesn’t exceed the allowed number of days, but it is always better to fill in P85 in both cases just to make life simple.

In Summary declare Non residency in UK when one moves out to benefit the tax free income, don’t mess with HMRC, they will catch you one way or the other
Keep it straight & simple