Have you got a friend who will say that you work for her as a cleaner/ dog-walker/ baby-sitter or whatever, for the extra hours, to make it up to the sixteen hours? You have to make sure it's at minimum wage so it doesn't affect other tax stuff, but it's been done before and worked...
I had the same problem as you last year - job was fourteen hours per week, and I asked them to put it up to 16, for no extra pay, as I would be better off. No questions asked, they did it.