When a mortgage deal ends, most loans move to the lender’s own variable rate, which the lender can change at any time.
Many lenders let you secure a new deal up to six months before the old one ends. Starting then gives time for a valuation and an offer, without paying a charge for leaving early.
Your current lender will usually make you an offer too. Compare it with the rest of the market, including fees, before you decide. General information, not advice.