If your business model includes import and/or export you can choose if you submit the custom declaration by yourself or by an agent. The gov.uk website announced a list of different kind of courier companies and customs brokers and agents who specialises in submitting declarations. See the list here. In addition, if you provide these services, you can list your own business.
The website states that the companies on the lists are no approved by any government body, therefore you must not forget to do robust bona fide and due diligent check on the provider as they are becoming your supplier.
In addition, if you are providing there services, you maybe eligible to apply for some financial help in the Customs Grant Scheme