Like every fire safety measure in a buliding, there is no explicit requirement in the actual Fire Safety Order for wardens- the legislation merely sets out the aims to which every premises must aspire to - suitable means of escape, suitable fire warning, etc.
The order requires to to carry out a suitable and sufficient Fire Risk Assessment & Emergency Plan so that you can decide the most appropriate measures to meet the basic requirements of the order, which is where the detailed measure is determined, such as number of exits, types of escape fastenings used, etc, etc.
The official guides to the Order set out in more detail what is considered as minimum suitable measures in a premises - these measures are used as a benchmark by the courts and enforcing bodies. You may deviate from these(hence why you risk assess rather than use the guides as prescriptive documents) , but must be able to demonstrate why your different approach achieves an equally suitable level of protection or you will be in breach of the Order.
The guides state that in large premises fire marshals/wardens normally form part of an evacuation plan. If you have a premises of a large size (which for evacuation could be anything over a small single storey unit) then the benchmark standard suggests you have wardens. If you do not and cannot provide a justification or alternative that will provide safe evacuation & confirmation of such, then you would be in breach of the order.
In short - legally unless you can justify otherwise in your FRA & Evac plan (could be size, use, occupancy reasons) you would be expected to provide wardens or an equivalent system.
