If the rear sight isn't windage adjustable then you will have to aim off; even once it's perfectly zeroed...I've had to aim at the edge of the target frame before at 600 with the Mauser due to the wind - With the windage on the SMLE, if you're not fussed about shooting it 'as original', you can put an adjustable rear sight on (assuming your's is a model without an adjustable rear sight)...it's very easy; just pop the pin out at the base of the sight, insert the replacement, and pop the pin back in...

Edit: Just thought I better point out that the point of aim will change with different loads and/or factory ammo...I had decided on a load to stick with before going through the process...