Enforce company number format

As per [this page](https://www.informdirect.co.uk/company-records/company-registration-number-crn-what-is-it/#:~:text=What%20is%20the%20format%20of%20the%20company%20registration,with%20the%20letters%20%E2%80%98SC%E2%80%99%20followed%20by%206%20digits.) company numbers are either 8 digits numbers or set combinations of 2 letters with 6 digits