In Eurocode 3 (EN 1993-1-1), the initial imperfections (global initial sway imperfections and relative initial local bow imperfections of members for flexural buckling) are replaced by equivalent horizontal forces. The values of the horizontal forces depend on the vertical loads. For example the global initial sway imperfections (φ) are replaced by a horizontal force HEd=φ*NEd, where NEd is the design value of the axial force. I am not familiar to British Standards but the procedure followed by Eurocodes seems reasonable to me. As for the Singapore's NA method, it looks similar to BS. Am I right?