It doesn't sound like a Whitworth to me. I've never seen one with an octagonal barrel or double set trigger.

On what basis have you concluded it is a Whitworth? Are there any such engravings on lock or barrel; eg. Whitworth Rifle Co. Manchester or Manchester Ordnance & Rifle Co?

The hexagonal bore is not exclusive to Whitworth and similar forms will be found in patched ball rifles. With the deep curving butt with long extensions on the comb and toe this sounds more like a bench rest or offhand rifle.

David