Welcome to Armorama, Roy
I can't recommend this site more.
Now, as for British Military in N.
Good luck. :-)
No, really. I model in N Scale myself. Post Modern American Military. (Future Military Rail)
I have to say that what you are looking for is VERY hard to find.
Here in the States, I would recommend that you send an SASE (Self Addressed Stamped Envelope) to:
J&M Hobbies
209 Old Post Dr.
Hauppauge, NY, 11788.
They will send you a catalog.
They do have a big assortment of modern AMERICAN Armor, Vehicles, Buildings and personnel.
I would say a lot of this stuff is interchangable and although I don't have the catalog in front of me, I wouldn't be suprised if there were Challengers in there.
I wish I could find a website for these guys, but the one I used to have doesn't work anymore.
Hope this helps and again, Welcome to Armorama!
Cuhail