Variable rate mortgages are based on the standard variable rate offered by mortgage lenders. The level at which the standard variable rate is set varies between different lenders. Each lender publishes their own rates as per their schedule of tariffs. However, don't assume lenders automatically shift their mortgage rates in line with market competitiveness.
Normally you can overpay on a variable rate deal - this is a good way to cut down the term and total interest on a loan. Compare different lenders rates and tariffs before making any decisions.