Ever since the famous words "don't worry, there's not going to be a hurricane..." were used, general UK forecasts have largely erred towards the poor side, IMHO. I can see why in that the fuss over predicted bad weather that turns out fairly benign is far less than that over unforecast downpours, storms, snow, etc.
In the quoted example, there's also a five hour gap between the old and new forecast, in which the CBs have gone as well as the winds associated with them. Overall, the accuracy doesn't seem too bad, considering they have to forecast what the worst likely conditions are going to be (visibility, cloud ceiling, wind, SIGWX, etc.) during the period. There are also tolerances involved - I don't know what they are but they might be in ICAO Annex 3, if you want to download that for £75...
Also, in terms of commercial operations, are the two TAFs that much different? Wind is not much off the runway when stronger, min. vis. is 4Km in RA in both and cloud ceiling is 5-600' vs. 700'.
It reminds me somewhat of India: there must have been an unforecast thunderstorm in 1953 because the TAFs generally include a TEMPO TSRA for the whole period at the end, even when there is no weather at all in the rest of the forecast!