Hi Victrix,
IMHO the only part of your Jezail which COULD be European is the barrel. The rest appears to be typically Afghan/Northern India. The area was called the Northwest Frontier by the Brits.
As has been stated above, these guns were often made up from parts obtained from, and stolen from, the particular Colonial Power which held sway in the region. Also local gunmakers were (and are still) very much able to reproduce copies of guns. There are many good copies of Martini Henry and much more modern rifles (AKs) around which have been made in the back streets of general area.
I am of the opinion that if a British Serviceman was to want to take a "souvenir" home, he would not go to the trouble of have a gun custom made, but would rather just "acquire" one from a local Tribesman.
The lock on yours also appears to be a copy rather than the genuine item. Although the strikings are good, the letters TOWER are slightly out of line, which would be unlikely on the real thing. Perhaps you could detach the lock and post some pics of the inside?
Your Jezail IMHO is a good representation of the type, and the fact that it is likely a "local" gun, no way detracts from it's interest (or likely value).
As an aside, the lock on my Jezail without the trigger guard was the subject of discussion here
http://www.vikingsword.com/vb/showthread.php?t=23563
Hope the above helps.
Stu