There may be times when you do not have the full registration number of a motor vehicle of interest, this is where a site such as partialnumberplate or lisenseuk can be useful, as they allow you to search a partial UK vehicle registration number. If you replace any unknown / missing characters with a ?, then it will return a list of possible registration numbers for vehicles, along with the vehicles make, model & colour.
If you are only missing a maximum of 2 characters, anywhere on the registration number, then these free online resources may be good enough, however both sites offer a paid for service if you missing more characters or want to filter by make, model or colour.