there are indeed a couple of agencies with London offices, who conduct interviews back in the UK for QP and for Qatar Gas. I know for fact that LA Recruitment does it.
Format is normally going through your current jobs (your duties, responsibilities), then past jobs, then... very often - about your personal circumstances (i.e. family, marital status, children etc). Watch out for questions about "why you wish to come to Qatar" - I saw people loose out just because of their answer to this question. Good luck!