The simple answer, is that to register as a doctor in the UK you will need to score 7.5 overall and not score below 7 in any part of the test. After that you will be able to sit for the PLAB test before registering as a doctor.
For more information visit the GMC website.
In Canada the IELTS band score needed to register as a doctor is lower. In Canada to register as a doctor you will need to score 7 overall and no lower than 7 in each of the papers.
In Australia, the situation is the same. To work as a doctor here, you will need to obtain an overall band score of 7 with no score lower than 7 in each component. However, there are rules around how many times you sit the IELTS exam in a 6 month period.