Your Jim Ainsley would have been 33 at the start of the 2nd world war in 1939.
Leaving plenty of time for James Marley Ainsley to be the same man that had the licence for Middlesbrough Hotel in 1934 (aged 28) and 5 years later go off to WW2.
"“Gunner” Jim Ainsley, well-known Hartlepool boxer of the 1930s, has died suddenly at home. Jim, 74, was cruiserweight champion of the North-East and was one of the best known boxers of his day. He fought more than 600 times, including bouts against such top names as Len Johnson, Bobby Shields and Jack London" From Mail, February 22, 1981.
Read more:
http://www.hartlepoolmail.co.uk/news/local/royal-seal-of-approval-1-1007 391#ixzz49Mnohgza
Gunner Ainsley was his nickname? Rather than his military rank?
Hartlepool Northern Daily Mail Durham, England
21 Feb 1931
GUNNER AINSLEY
"GUNNER AINSLEY in the country, and never fails please with his rugged, tearaway style. B? Hamilton? takes the place Billy Forrest (North Shields), and, Judging from his advertised record, should prove stern test for Ainsley.
((My own grandfather was over 30 years old in WW2, don't think it was only young, young men that went))
Photo
https://www.flickr.com/photos/hartlepool_museum/5754579910Could James Marley Ainsleys father or Uncle Ainsley have served in WW1? Have you got photographs of two different men here? One man in WW1 uniform and his son in WW2 uniform?
In the first photograph the man is wearing puttees, typical of WW1.
In the 2nd photograph the uniform looks more WW2, not to mention the umbrella in the back ground.