I would imagine that there is a little more to this rejection than just the positioning of the tattoo.
BA have a large number of mixed fleet crew who have tattoos which many would consider visible. Numerous crew have tattoos on their wrists, these are covered by plasters. Others have tattoos on their ankles, which again are either covered by plasters, or in some cases a heavy concealer. Either these individuals lied on their cv's or BA are very inconsistent in their requirements.