Hi Gary,
Few things to consider here -
It doesn't matter what type of mortgage you have out any application for a top up will be mainly judged on your ability to repay the extra amount regardless of value of the house. If you don't qualify for the amount needed now how will circumstances change in one year.
If you would qualify but are gambling on house prices rising to in effect get a 110% mortgage the banks will not easily lend for a top up unless the LTV ( loan to value ) hits 80%. Even then they will be looking for it to be for home improvements ( get a few quotes for a conservatory etc to get around ) Top ups within the first couple of years to pay off existing debt are rare.