Yes it is.
The only alternative would be the Birmingham proof house---which is a very different crowned "BNP".
Placement is also a clue.
Bolt body---Birmingham marks underside of the knob. London marks top of the handle.
Receiver---B. marks left side of receiver ring, L. the right.
I believe the same thing holds true for barrel mark positions, but am unwilling to pull out a bunch of rifles to confirm this.

The Arm is most often poorly struck.
-----krinko