When a fixed term ends, your loan moves to the floating rate unless you choose a new term. Lenders usually send a refix offer a few weeks before, but the first offer isn't always their best, and it's only one lender's view.
At the end of a fixed term there's usually no break cost, so it's the cheapest time to review. Compare refixing with your current lender against moving to another, counting any cashback you'd have to repay, legal fees and any new cash contribution.
Think about structure as much as rate. Splitting the loan across two or three terms spreads the risk of rates moving, and keeping part floating or on revolving credit lets you make extra repayments without break costs.
Start a month or two before your fixed rate ends. A refix reminder means you won't roll onto the floating rate by accident.