Just to add that if it's too much of a jump to go from interest only to repayments you can do it by increments - I think we went from a 50/50, 75/25 then 100% over the course of several remortgaging ocassions. Santander didn't charge two lots if arrangement fees for the different products. Probably not as unusual as you might imagine.
A word of warning Your mortgage lender will be very keen to move you to a repayment mortgage, but it is unlikely thst they will let you swap back if you want or need to; in fact a request to move from repayment to interest only apparently triggers a 'financial difficulty' alert on your account (the advice given to me at Barclays). As a pp suggested, if you may neec future flexibility, it may be better to make regular overpayments and stick with interest only.