EAIG16400 | Who to assess: where regulations do not specify the person liable to pay the duty
From HM Revenue & Customs · Excise Assessments Interim Guidance
In the few cases where regulations fix an excise duty point but do not specify the person liable to pay the duty at that duty point - see, for example, regulation 4(2)(f) of the Excise Goods (Holding, Movement, Warehousing and REDS) Regulations 1992 - you will need to choose the person to assess.
This should simply be a matter of deciding on someone with sufficient connection with the goods at the duty point. Where there is more than one person, you may need to make a decision based on who you consider most culpable.
As you may be called upon to explain why you chose to assess a particular person, you must be able to reasonably justify your choice.