AFAIK the 140 year is just an arbitary value the British Library have used, it isn't part of copyright law. The UK has used a life + X[1] years system since the

AFAIK the 140 year is just an arbitary value the British Library have used, it isn't part of copyright law. The UK has used a life + X[1] years system since the 1840s[2]. I guess they assume 140 years is enough to ensure any unknown / untracable authors have been dead for long enough for a work to be near unquestionably in the public domain. [1] Where X has steadily increased with each copyright law revision. [2] The US was a weird outlier on this for a long time after most of the world settled on this system.