Six digits with no label could be several things, and people arrive at a search like this one having copied a number off a document without noting what it was. Before anything else it is worth establishing that you are looking at a sort code at all, because the mistakes that follow from confusing one six digit field with another are tedious to unwind.
Is it a sort code in the first place
A UK sort code is six digits and is normally printed as three pairs. Other six digit numbers that turn up in the same documents include the opening block of a card number, a customer reference, a branch reference used internally by a bank, and a building society roll number fragment. Context settles it: a sort code appears beside an eight digit account number, usually with the account name, in a header or a payment details block. Standing alone with no account number next to it, six digits are ambiguous.
What the digits can support
If it is a sort code, the leading pair places it in a range allocated to an institution and the remaining digits identify a clearing point where that distinction still applies. There is no checksum to test, so nothing can be validated from the digits alone. The one derived fact available is which modulus rule applies to an account number quoted beside it, and that only becomes useful once you have the account number too.
Where the institution name comes from
Not from here. The register mapping codes to institutions is a licensed commercial dataset and this index does not republish it. What it does publish is each bank's own statements about its own identifiers, quoted from the bank's page with the date they were read, so that every entry can be traced. Where a bank publishes nothing, the entry records that, which is a more useful answer than a confident row from a table with no date on it.
What to do with the number you have
If it came from a document about an account you hold, open your banking app and compare it with the account details screen, which is current in a way a document is not. If it came from somebody who wants to be paid, ask them to confirm the sort code, account number and account name together, and let the name check answer when you add the payee. If it came from nowhere you can identify, do not use it for anything.
Questions people ask about 090129
How do I know if six digits are a sort code?
A sort code sits beside an eight digit account number in a payment details block, usually printed as three pairs. Alone, six digits are ambiguous.
Can I validate a sort code by arithmetic?
No. A sort code carries no check digit. The arithmetic check in the UK system applies to the account number, using a rule chosen by the sort code range.
Is a sort code the start of my card number?
No. The long card number is a card scheme number and has nothing to do with the sort code, even though both are printed on bank products.
Where can I confirm whose code it is?
Ask whoever gave it to you and check against that bank's own published pages. There is no free public register of UK sort codes.