im sure if you explain your situation more clearly to them, they can offer you some sort of help.
dont ask if you can swap to interest only, permanently, ask if you can just pay the interest payments for the next six months or so.
they wont let you swap to interest only unless there is an incentive for them to do so, for example, lend you extra money and charge you an arrangement fee etc.
there are many ways you can get help, the government have come down quite hard on lenders who refuse to help their clients, so call them and explain everything and ask them what they can do for you.
also give citizens advice a ring, these people are often overlooked, but are a great help.
good luck.