To answer my own question, I have found and purchased the following profile: 'Tamlyn XT-VERT'.
Available from here, among other places: https://guttercentre.co.uk/aluminium-trims/10077-896052-aluminium-vertical-profile-x-3m-length.html#/952-colour-mill_finish_non_painted
I'm having it powdercoated to match the Velfac windows, will report back after it's all installed.