Hiya Laurel,
It is possible that John Cullen was born in Ireland and joined the Constabulary in Ireland before joining the Police in England. I have borrowed a book from the library. It is by Jim Herlihy and is called "The Royal Irish Constabulary. A Complete Alphabetical List of Officers and Men, 1816-1922" The name and registration number enable you to find the service record on microfilm. These are available at PRONI, Balmoral Avenue, Belfast; the Garda Museum and Archives, The Round Tower, Dublin Castle and The Royal Ulster Constabulary Museum in Belfast.
7220 John Cullen
11236 John Cullen
13198 John Cullen
16191 John Cullen
19668 John Cullen
34051 John Cullen
35626 John Cullen
39816 John Cullen
42548 John Cullen
44958 John Cullen
50253 John Cullen
50697 John Cullen
52497 John Cullen
56071 John Cullen
60557 John Cullen
80626 John Cullen
62923 John T. Cullen
All the Best, Chris