Although the Bank Base Rate is likely to fall again, mortgage lenders are unlikley to pass on much (if any) of future falls. The deals they offer are linked to LIBOR and they need to make money. I feel many of the Banks cuts last week were pushed through by their new government shareholders and I am unsure this will happen again as the banks needs to make a decent margain on their mortgage books to rebuild their cash reserves. Therefore, I don't see fixed rates getting much lower than present levels.
The main concern with any mortgage is affordability and not the interest rate. I would be slightly concerened you are going interest only. I hope you have some form of repayment vehicle or are making savings to repay your mortgage loan. Look at what the payments might be after the fixed or initial tracking period.
I feel house prices have at least 15% more to fall in the next year. I would recommend you seek professional advice from an independent mortgage broker. They are not very busy at the moment so they should be keen to help you.