CFM82240 | Old rules: convertibles pre 2005: return on the security
From HM Revenue & Customs · Corporate Finance Manual
Return on a convertible security
This guidance applies to periods of account beginning before 1 January 2005
Any return on the security that came in addition to the shares had to satisfy both of two conditions.
The main aim of those conditions was to ensure returns that were in the form of a discount came within loan relationships, not chargeable gains.