Like you, I have (in the main) interest only. You don't say if you have endowments in place. I believe this is asked by the lender, but as we all know, there can be shortfalls in any case.
I have basically mixed and matched my mortgage. Some - the bulk - is on intersest only but small chunks are capital and interest, on which I overpay. This way I am keeping my payments low, but paying what I can on the capital and interest to reduce the sum borrowed. I've already settled one chunk this way, and converted a bit more of the remainder to capital and interest.
Why do you have to keep the same lender? If you are tied in to a mortgage because of some deal, then wouldn't you be tied to the same mortgage as opposed to the same lender.