Soffit boards under the eaves of pre-2000 homes can be asbestos cement or asbestos insulating board (AIB), which HSE lists among common locations (HSE). The difference matters: work on AIB normally needs an HSE-licensed contractor, while some work on asbestos cement can be non-licensed (HSE).
Replacing with uPVC
Many homeowners replace old soffits with uPVC. Before work starts, have the existing boards tested so the installer knows whether they can be removed by a non-licensed firm, need a licensed contractor, or can be safely covered.
Don't
Don't drill, cut or break soffit boards until you know what they are.