I will just point out to the OP that some of this stuff isn't 100% true.
Form P85 - Look at the P85 form, it says that if you are normally completing a UK SATR then you do not fill in a P85. As a UK contractor you are 99.9% certain to be doing a SATR so P85 is not required.
UK and overseas tax residency - It is true that for the first year of being outside the UK for tax purposes you will be entitled to split year treatment. But there are conditions attached. The main one being that you must be overseas tax resident for the following full tax year or you will automatically not qualify for split year treatment. For example - Leave the UK in March 2017 to qualify for split year treatment in 16/17 you must be out of the UK tax net for the whole of 17/18 as well. IE until April 5th 2018. Regarding qualifying for split year or none UK tax residence there are now the "automatic tests". Three layers of them. If you pass the first test that is the end of the story, you are qualified for outside UK tax status. Take a look at the first "automatic outside" test, it is not so hard to pass if you are genuinely full time employed in a foreign country. Look also at Form SA109 which you fill in to claim the split year treatment along with the SATR. There are software packages available that walk you through the returns and are pretty cheap, you may well not need to go to the expense of engaging a specialist tax adviser to fill in the SATR and SA109.
Hope that helps, but to the OP - DYOR.