I think you have to press button A while releasing button 2 four times anti-clockwise in sucession using only your left hand on a Tuesday. Of course if the display was previously set to metric and/or Southern hemisphere time, button A is in the position of button 1 and the polarity of button B is reversed. Left and Tuesday is fixed, while anti-clockwise only changes south of the equator, anything else would be trying to cram too many functions into too few buttons and therefore silly!
Leave it set to BST and subtract one in winter!
Andy