There is no regiment for him. There is one birth in Middlesex for Apr/Jun 1897, and that is for a John William Day, so given his RIC dob of 6 May 1897, this appears to be him. Also the RIC and the London Regt man are same dob
1897 May 6. Born Middlesex (from RIC & from officer file)
1901 census living in Ealing
1911 census living at 63 Bedford Road, West Ealing W
1915 Nov 25 Enlisted
1916 Jul 11 Commissioned 2nd Lt
1917 Nov 15. Wounded
1918 Nov 1 Promoted Lt
1919 Nov 6 Disenbodied
1920 Sep 14 Joined ADRIC with service no 562. Posted to E Coy
1921 Mar 13. Resignation accepted.
1921 Apr 23 leaves UK for Nigeria . Milford Rd, Ealing. He is an Assissant
1921 Nov 18. J W Day gazetted out as Lt in 12th London Regt
1924 Aug 26 Applies to Crown Agents for a Colonial Job
1925 Leaves UK for Nigeria
1930 Apr 6. Arrives UK from Nigeria
1931 Jun 12 Arrives UK from Trinidad - he is a government officer
1940 Sep 6 Leaves UK for Lagos. UK address is Ealing.
1946 Jun 13 Arrives UK from Nigeria. Colonial Official
1946 Dec leaves UK for West Africa - Civil Servant