Great Britain plus Ireland is one of the most difficult places in the world for medium and long term weather forecasting, if not the most difficult. It can be influenced by any one of six very different air masses, sometimes more than one. Autumn is particularly difficult as the remnants of hurricanes can provide a lot of energy into the North Atlantic that can easily perturb the jet stream completely from a hitherto set pattern.
The city forecasts are automatically produced by computer based on a particular model with no human intervention. The Met Office generally favours its own model whereas other providers may be working from something else. Go out a few days and they can be completely different, although if the models converge they will be similar.
Accuracy of forecasts can mean different things. Taking an example I knew I had to be out last Wednesday morning, and when I glanced at the forecast about five days before it suggested it would be damp then for a few hours. In the end it was dry - but it was damp about 4 hours earlier overnight. Whether that was good or bad depends on how you looks at it (forecasts each day put the wet spell a bit earlier, and the one 24 hours out was spot on).
Watch the forecast at 21:55 on the BBC News Channel - this covers the week ahead, and will give the confidence level, to the extent of sometimes two different scenarios from models.