Images: http://imgur.com/a/ce3MY
I hope this is the correct forum for this question. Thank you for any help you can provide!
Hallmarks are crazy difficult to determine with any certainty. I would suggest contacting a bayonet collector to find out for certain.
From what I can ascertain, it's either British or Canadian. It looks like it could be a Canadian Pattern 1888, but the hallmark seems to indicate it's British. WD with the arrow likely indicated War Department. The Crown with the E likely stands for Edward VII, which would date it to the 1900s, as Edward reigned from 1901 to 1910. But if could also stand for Enfield - the place it was made. The other crown BB is probably an inspectors stamp, but I cannot say for sure. And finally, there is some writing at the base of the blade that I cannot make out.
If I were you, and no one else here is able to give a definitive answer, I would take some better pictures (especially of the hallmarks - and don't forget the ones that should be on the scabbard and the grip) and then email them over to these guys. They will likely be able to give you a definitive answer, or at least point you in the right direction.
Good Luck.
The WD is for War Department which stopped being used in 1895, The E is the Enfield Royal Arms Factory inspection mark and the BR is the Birmingham R.S.A.F repair mark.
I suspect that may be an artillery pattern bayonet for a Martini Henry, like this one
http://www.martinihenry.org/index.php?route=product/product&path=61_64&product_id=82
How long is it? The guard is quite distinctive, not found on typical infantry bayonets nor similar to those on the cutlass style naval bayonets.
Thanks for all your help! The bayonet linked by /u/jonewer looks like it's the one.