A regiment comprised two battalions and a depot. The depot was in Northampton; while the 1st Battalion was in India until 1910. I don't know the exact movements of the 2nd Battalion but they spent some time in South Africa. Men would start in the depot then be transferred to one of the two battalions. But they could be transferred again later. He must have been in the 1st Bn for the Tirah campaign which finished on 31st January 1898.
The only way that he would be in Essex later in the year was if he was either on furlough; or he was transferred back to the depot; or to the 2nd Bn if they were in the UK at the time. Or he may have been transferred to another regiment. Does the marriage certificate say that he was still in the Northants?
Only by finding if his papers survive in Kew will you know the truth. If you cannot get to Kew Findmypast have scanned all pre-WW1 papers and will be publishing them 'by 2011'. If he served into WW1 his papers will be on Ancestry, else they were destroyed during WW2.
Men did not have a right to furlough. If he had less than say 7 years service (after the age of 18) I would discount that theory. With a less than common name you should check FindMyPast for more BMDs.
Another theory depends upon his age. He should have been over 25 when he married. If so he may have elected to go into the Army Reserve after serving 7-8 years in uniform. Army Reservists were recalled in late 1899 during the Boer War. He may then have spent the last of his 12 years service back in India.
I put this on another forum last week. Conditions for being allowed to marry -
It was from the Kings Regulations of 1912 (amended to 1914), but it would have been similar if not the same in his day. It shows the efforts the army took to ensure soldiers only married 'good' women:
KR 1360 (1912)
Permission to marry will not be granted unless:-
(i) A vacancy exists on the married establishment.
(ii) The CO has satisfied himself as to the woman’s character.
(iii) The soldier, if below the rank of serjeant, has
(a) £5 in the Post Office Savings Bank
(b) 7 years’ service exclusive of boy’s service
(c) Two good conduct badges, or, if a corporal, fulfils the conditions for two good conduct badges.
Ken