Does this help? The 'curvy' grid is on compass directions relative to true north, the square grid is what OS use for GB mapping...
(From p35 of this doc: http://www.arrows.demon.co.uk/wgs84/50000026.pdf)
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If you look at an OS map, it has info on it about the direction of true and magnetic Norths for each corner of the map...
So yes, on OS maps there is an angular offset that varies depending on where you look... BUT it's just a result of the projection used in OS maps. You could use a different projection which keeps north 'up' over the whole map, but then distances would look distorted instead - Scotland would be wider, south coast narrower. (There's always a distortion when projecting the 3D world onto a 2D map, but you can choose between distorting angles or distances, or both).