As long as you didn't get your PhD from an English speaking country, you are required to have IELTS certificate. They don't care whether your PhD was written in English or not. Let's say, you are from Egypt, you did your PhD in the Netherland (which is not an English speaking country ALTHOUGH courses there are conducted in English), when you get a postdoc post in the UK, you will be required to produce an IELTS certificate. They are very strict on that. If you are thinking of applying for a Postdoc, my advice is to sit for IELTS in advance even if you have not received an offer.