Confirm on the Bren gun- Its in Great Bend, in the chasm, all the way to the East, you'll find a raider that's been blown to hell. He's got the Bren and some other things. I think there is a difference with the ammo. THe Bren uses .303 ammo, so if you are carrying around excess ammo, this is a good time to use it up.
As for the enfield- you can cross the bridge and the moat. Alternatively you can cross the river with the APC and then crawl along the banks. Be careful of mutants with .50 cals as they will blow the APC to shit. However, you can creep the APC almost up to the top of the plateau. Be careful not to get stuck in the river though. It's not that hard, cross it like you would a trench.