As a rule I was always told they'd not made it past the Humber yet. I live in south Yorkshire directly inland from the Humber and we don't have them here. Though they do have them about 10 miles south of Doncaster.
I have heard reports of one been seen on the side of loch ness in the foyers / Gorthlec area. Was told this by a local resident who was adament it was a muntjac.

I have to doubt they have spread all that way North. Surely they are nowhere near the Scottish borders, they havent even got to parts of the South yet. If so they must have been given help.
It will be interesting to see how they fair in the colder Northern climate how far will they go?
And also how long will it take for their breeding to become seasonal?
No expert on Muntjac but was told by others that the climate was too harsh up here for them to survive?