Thank you for your post.
A person may make a Customs declaration in respect of goods if the person is able to present goods to Customs on import or if they are able to secure that the goods are presented to Customs on import. For many types of declaration however the declarant has to be UK established, in which case a non-UK established person would need to appoint a UK established agent to make a declaration. If two or more persons are capable of making a declaration for goods they will need clear agreement about the responsibilities of each when making the declaration. On who is liable the answer is set out in guidance here
. Who is liable The person or organisation who made the customs declaration relating to the goods being imported is liable for the customs debt (the debtor). If you are the declarant but use an agent or representative to make a customs declaration on your behalf, they may be liable depending on the type of representation.