Claims can be heard against employers who are based outside the UK, as long as you are ordinarily resident in the UK. If you are a citizen of another EU country posted in the UK, you can claim protection under UK law and submit a claim to an employment tribunal or court. Similarly, if you are a UK national working in another EU country, you are covered by their employment protection and can make a claim in their courts. The law is not straightforward and you will almost certainly need legal advice.
If you want to bring an employment claim in the UK employment tribunal and you are based outside the UK, the usual test that the tribunal applies is to ask whether you have a 'sufficiently strong connection' with the UK.