This is a fairly complex area of employment law. As a general rule, you cannot claim unfair dismissal in the UK Tribunals if you do not have any work connections with the UK.
It may be enough to work for a UK Employer, or if most of your working time is spent overseas but you are in the UK on occasions.
If you carry out some of your duties in the UK it increases the possibility for you to claim in the UK Tribunals. If you have worked in the UK, but you are temporarily posted overseas, again it is much more likely that your rights can be enforced in our Tribunal system.