What your bank can and cannot do
A bank sells its own mortgages. That is not a criticism, it is the job. But it means the answer you get is the best product that one lender has, not the best product available to you.
It also means the affordability calculation you get is that one lender's. Lenders differ enormously in how much they will lend to the same person, particularly if you are self-employed, work shifts, earn commission or have any adverse credit. A decline from your own bank tells you what that bank thinks. It does not tell you what the market thinks.
The other limitation is criteria. Every lender has rules about the property, your income type, your deposit source and your credit history. Your bank checks you against its rules only.
What a broker actually adds
Two things, mostly. The first is range: we can approach lenders you cannot walk into, including ones that only accept business through intermediaries.
The second matters more and gets talked about less. It is knowing which lender will say yes before you apply. Every declined application leaves a footprint on your credit file, and several in a row make the next one harder. Matching your circumstances to a lender's criteria first is most of the value.
There is also the administrative side, which is unglamorous but real. Chasing the lender, the solicitor and the estate agent is a job, and it is the part people underestimate when a purchase starts to wobble.
Where the bank wins
Sometimes it genuinely does, and any broker who tells you otherwise is overselling.
If you are an employed applicant with a clean credit file, a decent deposit and a straightforward property, plenty of lenders will want you, and your own bank may well have a competitive product with a simple existing-customer process.
Some lenders also keep a small number of deals for direct customers only. They are not usually market-leading, but they exist, and a broker cannot access them.
And a bank will not charge you a broker fee. If your case is simple and your bank's rate is close to the best available, the fee may not buy you enough to justify it.
Where it goes wrong
The pattern we see most often is somebody who went to their bank, was offered less than they expected or declined outright, and concluded that they could not buy. In a good number of those cases another lender would have said yes on the same income.
The second pattern is the product transfer. When a fixed rate ends, your existing lender offers you a new one. It is quick and there is usually no valuation or legal work, which is genuinely convenient. It is just not automatically the cheapest, and almost nobody checks it against the wider market before accepting.
A fair way to decide
Ask your bank what it will offer. It costs nothing and it gives you a number to measure against.
Then have somebody check that number against the rest of the market, and be clear about what any advice will cost you before you proceed. If the bank's offer is close to the best available and your case is simple, take it. If it is not, you have found out cheaply.
What you should not do is treat one lender's answer as the market's answer. That is the mistake that costs the most, and it is the easiest one to avoid.