As Mathepac says, it will be in the Articles (or, if the Articles simply refer to the relevant sections in Table A, in Table A of the Companies Act). It's usually 21 days for an AGM and 14 days for an EGM at which no special business will be considered, otherwise 21 days for an EGM too. Note that 21 days usually means 24 days total notice (i.e. discount the day you post, the day after and the day of the meeting itself).