If a penalty of 0.45% will give you peace of mind for five years then go for it I say.
Interest rates will probably not rise by 1% over the next 12 months, but they will almost definitely rise by more than that over the next five years.
When variable rates do start rising, fixed rates will already have risen in anticipation so the option of locking in at 4.3% for five years will be gone.
They say nobody rings a bell at the bottom, but I would say that 4.3% is significantly below long term average five year fixed term rates so you are not getting ripped off. Best of luck and hope it works out for you.