Hello MurcieBoy
I am intrigued by your attempts at forecasting and look forward to your next (even more so if it's a rare event)! Might I suggest that if you have uncertainties such as you had over the high pressure, that you indicate that in your forecast, for example, you said:
You could in your forecast say you predict a pressure of 1040 mb ±5mb for London, then give the reason for your margin of error as a note below so people know why you have that level of uncertainty. Just a suggestion, helps make your forecasts that bit more scientific and also help to combat the doubting Thomases!
As to my thoughts on long term forecasting, I am doubtful that the same patterns will repeat going ever further (or for that matter back) into the future (or past), or if they do, then there must be some larger (or more accurately longer) cycles/rhythms in force that would alter their effect and so reflect climate change. All incredibly interesting though! I wish we had more detailed weather data going further back!