How SONIA Swap Rates Control Your Fixed Mortgage Price

Here's a question that trips up a lot of people renewing a mortgage: the Bank of England's Monetary Policy Committee meets only eight times a year, and between meetings the base rate simply doesn't move. So why did the fixed-rate quote from your bank change three times in the last fortnight? The short answer is that fixed mortgages aren't priced off the base rate at all — they're priced off a different, faster-moving number called the SONIA swap rate, and understanding the difference is the key to understanding why fixed and variable mortgages can behave in completely opposite ways in the same week.

SONIA — the Sterling Overnight Index Average — is the Bank of England's own benchmark interest rate, and the Bank is literally its administrator: every London business day, it collects data on real overnight loans between financial institutions and calculates the average rate at which they actually lent to each other. It replaced LIBOR, the older and less reliable benchmark that was based on estimates rather than real transactions, with sterling LIBOR settings permanently stopping publication at the end of 2021. SONIA is now the reference point for an enormous share of UK financial markets — the Bank of England's own figures put it at over £90 trillion in referenced transactions annually. Right now, SONIA itself sits close to 3.73%, tracking just under the Bank of England's 3.75% base rate, which is exactly what you'd expect since it's measuring overnight borrowing costs directly shaped by that policy rate.

That close tracking at the overnight level is precisely why SONIA on its own doesn't explain fixed mortgage pricing. What does is the SONIA swap market — specifically, an instrument called an Overnight Indexed Swap. Think of it as insurance against interest rates moving over a period of years. A bank funding a five-year fixed mortgage doesn't want to be exposed to whatever happens to interest rates over the next five years, so it enters into a swap with another institutional counterparty: the bank agrees to pay a fixed rate for five years in exchange for receiving the floating SONIA rate over that same period. Whatever fixed rate the bank locks in through that swap becomes the foundation of the mortgage rate it can profitably offer you — because from that point forward, the bank's own cost of funding is fixed, regardless of what actually happens to interest rates afterward.

That swap rate isn't set by today's base rate — it's set by what the market collectively expects the average interest rate to be over the life of the swap. If traders expect the Bank of England to cut rates repeatedly over the next two years, the two-year swap rate will sit below today's base rate, pricing in those expected cuts in advance. If they expect rates to stay elevated or rise, the swap rate sits above it. This is the entire reason fixed mortgage rates can fall even while the Bank of England is holding or raising its base rate, and why they can rise even during a rate hold — the swap market has already moved on updated expectations before the Bank's next meeting even happens.

From that swap rate, a lender builds the actual number you're quoted through several layers stacked on top. Using illustrative figures for a hypothetical five-year fix: start with the five-year SONIA swap rate itself, say around 3.25% — that's the lender's raw wholesale cost of locking in fixed funding. On top of that sits a credit and default risk reserve, roughly 0.35%, sized according to the loan-to-value tier and the lender's own risk appetite. Then a capital and operational cost layer, covering the regulatory capital the lender must hold against the loan and the administrative cost of running it, perhaps another 0.40%. Finally, the lender's own commercial profit margin, commonly around 0.50%. Add those together — 3.25% plus 0.35% plus 0.40% plus 0.50% — and you land close to 4.50%, the rate actually quoted on the high street. The exact split varies by lender and by month, but the structure is consistent: swap rate as the foundation, with roughly one to two percentage points of margin layered on top for risk, cost and profit.

It's worth being precise about what this does and doesn't apply to. Tracker mortgages and most variable and standard variable rate deals move directly with the Bank of England's base rate — when the Bank cuts or hikes, those rates typically follow within weeks, sometimes days. Fixed rates are the ones governed by the swap mechanism described here. This is also why the base rate decision itself, on the day it's announced, often has almost no effect on fixed mortgage pricing — the market usually saw it coming, and priced that expectation into the relevant swap rate weeks beforehand.

Two-year and five-year swap rates don't always move together, and the gap between them tells you something useful. Two-year swaps are highly sensitive to near-term expectations — the next one or two Bank of England decisions, the latest inflation print, anything that shifts the near-term policy path. Five-year swaps reflect a longer-run average expectation and tend to move more slowly, since a single data release rarely changes what the market thinks average rates will be five years out. When the two-year rate sits meaningfully above the five-year rate, the market is effectively pricing in higher rates in the near term followed by cuts later — sometimes described as an inverted curve. When they sit close together, or five-year is higher, the market expects rates to hold roughly steady or drift up over the longer run.

For a homeowner or remortgager, the practical upshot is this: base rate headlines tell you what's already happened to short-term policy, not what your next fixed quote will look like — swap rate movement, which happens continuously and often ahead of the news, is the thing actually driving that number. Most lenders let you lock in a rate three to six months before your current deal ends, which means the swap rate on the day you apply, not the day your deal completes, is often what matters most. If you're watching for a good moment to lock in, tracking swap rate trends over the preceding weeks tells you far more than waiting for the next Bank of England announcement — by the time that announcement happens, the swap market has usually already told you what to expect.