It doesn't matter, they look at your candidate number. Half of my exams say Beccy and half say Rebecca and there's nothing wrong because they catalogue everything using your candidate number so put whatever you want
no its not like that i changed my name in the school because i like to be called by my middle name
Put the one that is on the timetable given to you a few weeks before your exams start. As others have said, it doesn't matter hugely because they'll just your candidate number/centre number to ascertain who you are because, chances are, there's somebody else doing that paper with the same name as you.