Although most residential mortgages track the Bank of England base rate, some will follow Libor (London Inter Bank Rate) instead. This is the rate at which banks lend to each other and it can vary day to day, so there is more movement in it than the base rate.
A Which? Mortgage Adviser will be able to talk to you about the possible benefits and drawbacks of opting for a Libor mortgage. But you may prefer to opt for a tracker mortgage or another type of variable rate or a fixed rate mortgage deal instead.