Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
Firstly, when you quit these shores all those years ago did you complete a Form P85 and send it to the Inland Revenue as it was then? If you did not you should do so immediately, but now, of course, to HMRC. On receipt that Department will classify you as non resident from your original departure date. Once so classified you can spend up to 91 days in the UK in any one tax year without prejudicing your status. In theory the 91 days can be averaged out over a four year period, but the consensus of experts opinion on this site is never to exceed the magic 91.
If you are non resident in the UK you can forget about VAT until you finally return, when you should advise HMRC by letter, and then only need to register if your turnover in the UK any one year exceeds the 83K threshold. For the return year HMRC will split it into two portions, one non resident and the other resident.
I do hope that I have been able to set your mind at rest on this matter.
11/12 in UK over 183 days so liable to UK taxation on your world wide income. Any overseas tax deducted will be allowed as a tax credit if that country has a Double Taxation Treaty with the UK.
12/13 Same but check dates carefully; it is the dates you leave not arrive which matter.
13/14 Not taxable.
16/17 As for 13/14
Your activities would have been zero rated exports anyway for VAT so although you should have registered there would be no tax to pay. You could, of course, have reclaimed any input tax charged.
I would suggest that you consult a trusted, local professional to fight your case on this if push ever comes to shove with HMRC. That Department clearly have you recorded as non resident for tax purpose.
Thank you for your support.
As far as HMRC is concerned for your time away I would be inclined to emulate Brer Fox, 'For he lay low and say nuffin!'