There are two quite different versions of this question. One is somebody trying to find their own sort code, which is a matter of knowing where to look and takes under a minute. The other is somebody trying to find the sort code for a bank they are paying, which is a different problem with a different answer, and conflating the two is why the search so often ends badly.
Finding your own, ranked by how current it is
The bank's app is first, because it reads the live record. A recent statement is second and has the advantage of showing the account name and the bank's registered office in the same place. A cheque book or paying in slip is third and is fine if it belongs to the current account. A card is fourth and is increasingly blank by design. An old letter is last and should be treated as a hint rather than a source, because products get migrated and details change with them.
Finding someone else's is not the same task
You cannot look up another person's sort code, and you should be suspicious of any service that offers to. What you can do is ask them for it through a channel you trust, then let your own bank's name check tell you whether the name you typed matches the account. If a business has sent you details by email and something feels off, telephone them on a number you already had. Almost every large loss of this kind starts with details that arrived in a message rather than details that were wrong in a directory.
When the only source you have is old
If all you have is a statement from a few years ago or a cheque from a closed book, treat the pair as unverified. Banks migrate accounts between products and occasionally between codes, and a payment sent to a dead pair will usually bounce but can take days to come back. The safe route is to confirm with the bank before sending anything substantial, or to send a small first payment and wait for the payee to say it landed.
What to do when a form wants more than the pair
Forms often ask for the bank name and address alongside the sort code and account number. Use the registered office the bank publishes, which appears on statements and on the bank's own website. If it is an international form it will want the IBAN and the BIC instead, both of which your bank publishes on a help page and shows in the app. Filling in a branch address for a bank that has no branches is a common cause of a form coming back.
Questions people ask about find sort code
What is the quickest way to find my sort code?
The account details or share details screen in your banking app. It reads from the live record and usually lets you copy both fields at once.
Can I find a sort code from a company name?
Not reliably, and not from this site. Ask the company for its details in writing and let your bank's name check confirm them.
Is the sort code on a cheque reliable?
Yes, provided the cheque book belongs to the account you are asking about and that account has not been migrated since the book was issued.
What if my bank has no app and I have no statement?
Telephone the bank. They will confirm the sort code and account number after identifying you, which is the only route that needs no document at all.