Disclaimer:
I'm a UK expat, not a tax lawyer, so don't put much weight on what I say, but is what I seem to remember from my dealings with the inland revenue.
1. If yoiu stay in UK for more than 90 days per year on a 3 year rolling average (or more than 183 days in any one year) then you are a UK tax resident (you can end up being tax resident in multiple countries simultaneously) and your worldwide earnings (including investements) become subject to UK tax.
2. Regardless of your UK tax residence status, UK earnings are always UK taxeable, and tax (plus NI) will most likely be deducted at source.
There is a rule where a person's job can take place in many countries (such as a pilot). I can't remember the exact wording, but its something like significant or substantial 'part of the job'. I also believe a Pilot went through the courts with the argument that he only landed in (LHR) UK. The court decided that landing an aircraft was sufficient a part of a pilot's job that it made that individual subject to tax on his earnings. Obviously this cannot apply to foreign based pilots, so it may also depend where your company is based, or where your company says you are based.
If you have no choice about paying tax in the *other* coutry you have residence (ie you have to pay tax there regardless of what happens in UK) you might be able to claim relief under a dopuble taxation treaty, but this varies widely from country to country and might not exist with the country you are resident in.
Don't be afraid to call up the inland revenue (HM Revenue and Customes as they are now called) - without exception, every time I have needed to get in touch with them they have been fair and extremely helpeful. They are not your enemy - they just have the unfortunate job of administering and enforcing HM government policy on tax.
Your first stop for more info might be:
http://www.hmrc.gov.uk/leaflets/c9.htm
then download and read IR20 - and maybe some of the others. Look around other parts of the site - the leaflets are packed with info that is easy for us non-lawyers to understand.
Good luck!