Why Are Mortgage Rates Going Up When the Base Rate Hasn't Changed?
It's one of the most frustrating questions in personal finance right now. You've heard the Bank of England has held its base rate steady, yet you've just received a renewal offer that's significantly higher than your last deal — or you've noticed the best-buy tables creeping upward. So why are mortgage rates going up when the headline rate hasn't moved?
The honest answer is that the base rate is just one piece of a much larger puzzle. To truly understand what's driving your mortgage costs, you need to look at gilt yields, swap rates, SONIA, and lender margins. None of these terms tend to make the evening news, but together they quietly determine what millions of UK homeowners pay each month.
The Base Rate Is Not a Mortgage Rate
The Bank of England base rate is the interest rate at which commercial banks borrow money overnight from the Bank of England. It acts as a foundation for the entire financial system and has a strong influence on tracker mortgages and savings rates — but its connection to fixed-rate mortgages is far more indirect than most people realise.
When a lender offers you a two-year or five-year fixed mortgage, they're not simply adding a margin on top of today's base rate. They're pricing in what they expect the cost of money to be over the entire term of your deal. That forward-looking view is shaped by entirely different market forces.
Swap Rates: The Real Engine Behind Fixed Mortgage Pricing
The most direct influence on fixed mortgage rates is something called interest rate swaps. When a lender offers you a fixed rate, they take on the risk that interest rates could rise during your deal period. To hedge that risk, they enter into a swap agreement with another financial institution — essentially locking in a fixed rate in exchange for paying a floating one.
The price of these agreements — known as swap rates — reflects the financial market's collective forecast of where interest rates are headed. If markets believe rates will remain elevated or rise further, two-year and five-year swap rates climb accordingly, and lenders pass those costs directly into their fixed mortgage products.
According to data tracked by financial markets, UK two-year swap rates have remained stubbornly elevated even during periods when the base rate has been held steady, because traders are pricing in persistent inflation and a "higher for longer" interest rate environment. This disconnect is precisely why you can see fixed mortgage rates rise even when the Bank of England hasn't touched the base rate.
Gilt Yields: The Government's Borrowing Costs Affect Yours
Another crucial factor is UK government bond yields, commonly called gilt yields. When the UK government borrows money, it does so by issuing gilts. The yield — the effective interest rate investors demand to hold these bonds — acts as a benchmark for long-term lending throughout the economy, including mortgages.
When gilt yields rise, it signals that investors require a higher return on safe assets. Lenders, who also compete for the same pool of investment capital, must adjust their mortgage pricing upward to remain competitive and profitable. According to Bank of England yield curve data, UK gilt yields have remained at multi-year highs, creating persistent upward pressure on mortgage rates that operates entirely independently of base rate decisions.
Global factors play a role here too. Events such as shifts in US Federal Reserve policy, geopolitical instability, and concerns about UK public finances can all cause investors to demand higher yields on UK gilts — and your mortgage rate can move as a result, even if nothing has changed domestically.
What Is SONIA and Why Does It Matter?
SONIA stands for the Sterling Overnight Index Average. It's the interest rate benchmark that replaced LIBOR in the UK, and it reflects the average interest rate banks pay to borrow sterling overnight in the wholesale market. Administered by the Bank of England, SONIA is closely tied to the base rate and is most relevant to variable-rate and tracker mortgage products.
For most homeowners on fixed deals, SONIA has limited day-to-day relevance. However, it becomes important when understanding lender funding costs more broadly, since the overall cost of short-term borrowing in the wholesale market feeds into how cheaply — or expensively — banks can raise the money they lend out as mortgages.
Lender Margins: The Profit Layer on Top
Even if swap rates and gilt yields were perfectly stable, lenders have their own margins to consider. These are the percentage points added above their funding costs to cover operational expenses, bad debt provisions, regulatory capital requirements, and profit.
During periods of economic uncertainty, lenders often widen their margins to build in a buffer against potential defaults. According to the Financial Conduct Authority's Financial Lives survey, financial stress among UK households has increased in recent years, meaning lenders are pricing in higher risk — and that cost is reflected in the rates offered to borrowers.
Competition between lenders can compress these margins, which is why you'll sometimes see mortgage rates fall even when swap rates haven't moved — lenders are simply fighting harder for business. Conversely, when lenders have strong pipelines and don't need to aggressively compete, margins widen and rates creep up.
The Outlook: What Could Bring Mortgage Rates Down?
For mortgage rates to fall meaningfully, several things generally need to happen simultaneously. Swap rates need to come down — which typically requires markets to believe the Bank of England will cut rates, and cut them more aggressively than currently expected. Gilt yields need to ease, which usually means lower inflation, more stable public finances, and calmer global markets. And lenders need to feel confident enough about the economy to tighten their margins.
According to ONS inflation data, UK inflation has been on a downward trajectory, which has given the Bank of England room to begin cutting rates gradually. However, markets remain cautious about how far and how fast those cuts will go — and until that uncertainty resolves, swap rates and mortgage pricing are likely to remain elevated relative to historical norms.
If you're approaching the end of a fixed deal, it's worth speaking to a whole-of-market broker who can access products across dozens of lenders — ensuring you're getting the most competitive pricing available rather than simply accepting your existing lender's retention offer. You can also explore our remortgaging guide for a full breakdown of the process.
Making Sense of the Numbers
If you're a first-time buyer trying to navigate this environment, understanding how rates are priced is essential before committing to a deal. Our first-time buyer guide walks through the full process, including how to assess affordability when rates are in flux. It's also worth checking your loan-to-value ratio, since a lower LTV typically unlocks better rates — sometimes the decision to save a larger deposit can meaningfully improve your deal.