Go to the Office of Rail and Road website for background information on licensing - link below.

The term is used in English to refer to the authorisation given to someone to move on a particular street or road by car

Same ORR website referenced by Suzie and Bokani, but I believe this is the section that names the actual permit (namely a "safety certificate") required to *let the rolling stock loose on the public (*borrowing from Daryo!), and tantamount to a "homologation."
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"What is safety certification and safety authorisation?
Under ROGS, no one is able to operate vehicles or manage infrastructure on the UK railway unless they have obtained the appropriate **safety certificate** or authorisation.

**Those operating vehicles require a certificate,** and those who operate infrastructure require a safety authorisation. If a train operator also manages infrastructure (or vice versa), they will need both a certificate and an authorisation."
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In light of the above distinction, it seems appropriate that the train car manufacturer would be applying for a "safety certificate" in order to get their cars okayed for use on UK railway tracks.