Why do we have several exam boards producing GCSE and A level papers in the UK(excluding Scotland)? How can they possibly have the same level of difficulty? Surely it would be fairer to just a one single exam board?
We have more than one exam board because historically exams were set by universities. The issue does come up occasionally, most recently over concerns that different exam boards were competing for custom by being 'easier' than the others. The government probably doesn't have the capacity to set up its own centralised exam board so Michael Gove wanted to solve the problem by only allowing on exam board per subject, but that idea was abandoned, probably because deciding which exam board would get which subject was a huge minefield.
It's Ofqual's job to ensure that standards are maintained between exam boards, they approve the syllabus, exam papers and where each exam board sets its grade boundaries each year.