Posted By Sean Fraser
According to the HSE leaflet, it is anyone who is not under close or direct supervison and can apply to fixed establishments and mobile workers.

http://www.hse.gov.uk/pubns/indg73.pdf

The question is not so much that they are a lone worker, it is whether the nature of their work carries a sufficient level of risk that they are not given the same level of assurance that supervised workers have i.e. first aid. This should be considered during the Risk Assessment process as only one element of their activities. To me it is the same additional consideration that should be applied to younger workers and expectant mothers - an increased vulnerability that needs accounted for.