Just wondering
Your interpretation of the TAF is largely correct; very poor weather was forecast, i.e. the forecaster(s) believed that the storm was going to hit Glasgow; however what happened was that it wasn't quite as bad as they first thought, and it dissapated before reaching the West Coast.
Re: the point about the vis being not too bad in Glasgow and being worse than forecast in Belfast. Vis is notoriously difficult, if not impossible, to forecast correctly, especially when precipitation is involved; any estimate of vis in a TAF when RA or SHRA is involved is at best a stab in the dark. The vis might get very poor in a heavy shower, there again if the precip is light then the vis may stay well above 10k.
I can understand some of your points though, but the fact is that the weather didn't turn out as bad as it looked it was going to be. I don't really see the problem with that. Perhaps a tad pessimistic it could be said, but there again the forecast is always the absolute worst the Met thinks its going to be, with the data they have at the time.
Saying that the Met Office isn't fit for purpose is a little harsh though, perhaps a little too "Daily Mail". There are limitations to forecasting, we have to understand that and work with it.
Just my take on it...
Smithy