Davorka, the following is an excerpt from a Trade Compliance Handbook prepared on behalf of the US Customs Service. Unfortunately I am not familiar with the terminology used by the Britons.

A. Principal – This party must obtain a bond to ensure the payment of its obligations to Customs.
(Or as it was explained to by my better half the principal is the one who is ultimately responsible for the payment)

B.Surety – This party furnishes the bond. It is usually an insurance company approved by Customs. The surety agrees to pay the principals obligations (duties, taxes, penalties or liquidated damages) to Customs, should the principal default. The surety charges the principal a fee for this service.
(This might help you if you are looking for a name for these types of agencies)

C. Creditor – This party is paid by the surety if the principal defaults on its obligations. Customs is the creditor for all Customs bonds.