As per WMO convention the validity for SigWx charts is 6-9 hrs.
UK Met office revised the validity considering the flight duration times for most users. So, if the vaild time is 1200z it is valid from 0800-1700z [ 4hrs before and 5 hrs after] The issue time is of no relevance except for the overall picture as they are issued 12-14 hrs prior to vaild times.
http://www2.metoffice.gov.uk/aviatio...s/details.html