English certificate for postdoc in the UK
Being a Tier 2 migrant here myself, I can confirm that IELTS Academic exam is accepted for Tier 2 applications. The general rules are here.

However, a word of caution is needed. UKVI has recently changed the list of approved test providers, and IELTS franchise seems to split into "IELTS" and "IELTS for UKVI" providers. Only "IELTS for UKVI" providers are approved for immigration purposes. The exams are essentially the same, it is only the level of screening, security and scrutiny which is different.

It is not clear to me whether the  IELTS certificates issued before the split are still accepted by UKVI. Many people assume that they should because the laws can not be applied retrospectively, but IANAL. I would like to know the answer, too.