SolvedCA20080 | Plant and Machinery Allowances (PMA): introduction: parts and shares
From HM Revenue & Customs · Capital Allowances Manual
A part of an asset is treated as an asset for the purposes of the plant and machinery legislation CA11600.
The plant or machinery legislation applies to a share in plant or machinery in the same way as it applies to a part of plant or machinery. A share in plant or machinery is treated as used for the purposes of a trade so long as, and only so long as, the plant or machinery is used for the purposes of a trade CA29210.