The Open Transport Initiative UK API provides 2 x different Open Standard API specifications for free usage since v1.0:
A standard way to facilitate peer-to-peer transport data sharing and account interoperability, allowing the customer to combine all their transportation, mobility and associated data in one place. Including: purchase, concession, and usage features in addition to operators, transactions, latitude and longitude, vehicle, and discount data.
A centralised look-up for all transport operators. This provides a transport directory service with unique reference information about each mode of transport or mobility organisation, including any publicly available Customer-account API URLs.
Both are made available on Swaggerhub for usage without conditions or fees.