Hi Gareth,
The timeline for the 67th Regiment of Foot:
1865-1879
South Africa
Home Service. Ireland, England
Burma
India
2nd Afghan War (1879)
I noticed on the census entries William was still a soldier on the 1881 census but it doesn't give his Regiment however if he was still serving in the 67th Regiment of Foot it's possible he took part in the 2nd Afghan War of 1879 and saw action at the Battle of Charasiah.
The Timeline of the Regiment confirms the details of the birth of two of his children being born in India and Burma and except for the possibility he was in Afghan all the other postings would have been peacetime duties. The only other possible military engagement he could have been involved in was the Third Anglo-Burmese War 1885-87 2nd Battalion (1st Battalion also served during 1888, qualifying for the later clasp) but it depended when he was discharged between 1881-1891 based on the census information.
He would have been awarded the India General Service Medal 1854-95 for his service in India and depending on his postings maybe the Afghanistan Medal 1878-1880.
The 67th Regiment became the Hampshire Regiment (1st & 2nd Battalions) from 1881.
What you need to find out is information on his military records and I'm sure due to his length of service he would have been awarded a pension and these records should be held at the National Archive
http://www.nationalarchives.gov.ukCheck out this site for history on the British in the Raj which also contains a searchable database of records and documents that may help in your research.
http://www.fibis.orgThe nation archive also holds these Regiments records:
Record Summary
Title 37th Foot, 67th Foot and Hampshire Regiment
Covering dates 1877-1888
Held by The National Archives, Kew
Context : quick reference
WO Records created or inherited by the War Office, Armed Forces, Judge Advocate General, and related bodies
Division within WO Records of other administrative departments of the War Office
WO 16 War Office: Muster Books and Pay Lists
Subseries within WO 16 Infantry
Contact the Regimental Museum because they may hold some individual soldiers' records or they maybe able to help and advice but it depends on the individual museum however it costs nothing to check.
http://www.royalhampshireregimentmuseum.co.ukThe website Mark gave is well worth posting on because you'll usually find someone with a keen interest in the 67th Regiment and may have lots of good resources they're willing to share.
Please don't hesitate in asking for further help and let us know how you get on because if we can confirm the Afghan war then we can get lots of further information.
Good luck
Kind regards
Adrian