The short answer
If the car is UK-registered and you have the number plate in front of you, start with the registration. It is quick to type and usually the least fiddly route. If the car is imported, unregistered or wearing a recently changed plate, the VIN is the steadier choice.
Both can lead to the correct vehicle record, but they do different jobs. A registration belongs to an administrative record and can change during the car’s life. The VIN is the 17-character identifier attached to the vehicle itself.
When a UK registration is easiest
For an ordinary UK-market car, the registration lookup is wonderfully simple: enter the plate, review the returned make and model, then check whether a report is available. No crouching by the windscreen with a phone torch required.
Take extra care with cherished plates, recent transfers and private registrations. A single mistyped character can also point nowhere useful. The previewed vehicle details matter more than the familiar shape of the registration, so make sure they match the car you are researching.
When the VIN is the better anchor
Use the VIN when you want to identify the exact chassis independently of its number plate. That makes it the natural option for imported cars, vehicles outside the UK, project cars that are not currently registered and anything with uncertain paperwork.
Copy all 17 characters from the vehicle or an official document. Modern VINs do not use I, O or Q, which helps prevent confusion with 1 and 0—but reflections through a windscreen can still turn careful people into reluctant cryptographers.
Let the preview settle it
Whichever route you use, compare the returned identity with the actual car before paying. If the make, model or other details look wrong, stop and correct the input rather than assuming the identifier pattern has proved anything.
Report coverage still varies by manufacturer, year and market. The free preview is there to confirm both identity and availability, which is much more useful than debating VIN vs UK registration in the abstract.