I think I read somewhere that the london issued sigwx charts have a validity period of 4 hours before until 5 hours after the stated validity time.
The Low Level (< FL100) SigWx charts issued by London (Forms 215 and 415) have exactly that validity -- it's printed on the chart. But I don't think the Mid and High Level SigWx have any such validity window.
its not a snapshot of the wx at the particular valid time as it is issued ahead of time.
That doesn't follow. By "snapshot" all we mean is that only weather forecast to occur
at the validity time is shown on the chart. The Low Level charts will show weather forecast to disappear before or appear after the validity time for the fronts and zones as well. So for example today the F215 with fronts shown for 1800Z says "ISOL CB 020/XXX FAR N TL 17Z", because the validity window is 1400 to 2300Z. If it were a "snapshot" at 1800 it wouldn't show such weather.