Mk. VII standard .303 was a flat based 174 grain bullet. As you've already mentioned, it is a .312 diameter bullet as well.
Shooting a boat tail projectile out of an Lee-Enfield has the possibility of it not fully engaging the rifling, due to the somewhat reduced surface area (taper) of the BT, which can cause keyholes.
Now, you can use boat tail ammo in a Lee-Enfield and have good results, but it seems to be a luck of the draw sort of thing. If you've got a tight barrel, odds are you'll do fine. Two groove No.4s, not as much...