Lateral torsional buckling

If it is torsionally braced at 3m centers, I would say you should consider the LTB length to be 3m.

BA

In order to consider the cantilever to be torsionally braced, the magnitude of rotation may be critical, so the stiffness of the purlins becomes a consideration.

There does not appear to be a knee brace at the column, so the stiffness of the column itself must satisfy deflection requirements specified in the code in order to be considered an adequate lateral brace.

BA

I agree with BA's comments and would add that you should closely examine the warping restraint of the column if you are utilizing the warping strength of cantilevered beam. Adding stiffeners to the column would help the situation unless your column section is massive.