Prisoner recruits into British regiments

Hello Many soldiers on my family tree served with distinction, being awarded MC, MM, DSO, DCM. I also have a relative who was charged with murder, plead guilty to manslaughter, & on 11 Dec 1916 was sentenced to 5yrs imprisonment. However, 20mths later he was Killed in Action 24 Aug 1918 in France, serving with the King's Royal Rifle Corps, 12th London Regiment 'The Rangers'. I have a subscription to Ancestry.com but cannot find his Service Papers. Although, I have found his Medal Roll Index Card, & his Personal Effects entry. My question is: Due to declining numbers of volunteers, did the British Govt grant amnesties to prisoners prepared to enlist? And was it all kept hush, hush? As I have been unable to find any information on this.

Thanks for any assistance.