I read that the name Skylark was one of a long list of names submitted to replace "CTV5 Series 3". But what I'm really interested in is whether there's any connection to the "Skylark" series of sci-fi novels written by E.E. "Doc" Smith in the 1920s and 1930s.
According to Doug Millard's Skylark: Britain's Pioneering Space Rocket:
The name Skylark replaced the rather uninspiring CTV5 Series 3. A scientific officer working on the early programme felt the rocket needed a catchier name and so drew up a long-list of equally unlikely alternatives that ended with ‘Skylark’ – which was duly chosen.
Over almost half a century a total of 441 Skylark rockets were launched, making it one of the longest and most successful programmes of its kind in the world.