DEFRA does not have any rules about accompanied vs unaccompanied animals. As long as the airline agrees & there is someone to meet the animal on the other side with all the appropriate paperwork, etc., then it doesn't matter.
From the DEFRA website: "Owners of pets entering the UK from outside the European Community are required to complete Customs formalities. An agent, travel company or airline should be able to do this for you and you will probably be charged. Alternatively, you might be able to make your own arrangements. This would involve contacting the Customs authorities at your arrival point in the UK to obtain details of the necessary procedures and the appropriate Customs form.
You are advised to discuss these charges with your agent etc before you travel as they might be included as part of a package. These charges are not made by the UK Government."
(disclaimer: I work for DEFRA answering queries like this, but I am not a PETS official)