I don’t think that is a British 6.6” projectile. The picture below is of a page from David Ibbetson’s excellent book on British Artillery Ammunition, Volume 1, a must for the collector of early ordnance.
6.6” ammunition was not studded, but had a gas check plate attached to the base. The shell is not typical of British produced ammunition of the period in my opinion. The larger studs would require non standard riffling. The rough base moulding without markings and the wide, flat nose are not typical of British ammunition. If the orange paint is original, which it appears to be, that is typical of paint on early Prussian/German ammunition. I have a similar, smaller calibre projectile with the same studs which I am yet to identify.