See this sample document, OCN:
https://eservices.landregistry.gov.uk/eservices/FindAProperty/view/resources/example_register.pdf
Note that on the third page the following words appear:
"Except under an order of the registrar no disposition by the proprietor of the land is to be registered without the consent of the proprietor of the charge . . . "
That's an indication that there's a charge ( = mortgage) registered against the property.
Then look further down, on the last page, for where it says "Registered Charge" (circled in red). That gives details of the actual charge.
If similar wording appears on your own title register then the Land Registry is under the impression that you've not yet paid your mortgage off.
However if the
only reference to the Nationwide Building Society is akin to the one shown on the first page of the sample document (referring to the fictional Ilkingham Building Society), then it's purely historic information about who the lender was at the time of purchase.