Canadian External Examiner for UK PhD Viva advice
Many academics and other subjects experts from all corners of the globe are invited to be external examiners for PhD degrees. It is not uncommon, and the process is designed for this situation. In the UK it is also common that a viva panel is governed by a neutral chair who ensure that all the examiners are familiar with the procedures and regulation for conducting the examination and that the questions and conduct are fair for all parties concerned. 

Those panels that do not use a separate chair, then it is often the internal examiner that ensures the external understands the scope and process of the examination.

It is important for the (world-wide) standard of PhD's that the foremost experts at any particular field can be called on to participate in the PhD examination process. You should feel it a compliment to your work that the university reached out to find a suitable specialist to read and comment on your work. The fact that they are willing to cover the not inconsiderable expenses to bring this examiner to your viva demonstrates the regard they have for your work. It is a positive thing for you.