It can be mathematically proved that, irrespective of any future advances in meteorology, it will NEVER be possible to provide accurate weather forecasts more than a few days ahead. (It's called 'chaos theory'). All that the Met Office can do is to
(a) examine historical trends ; and
(b) look at the prevailing patterns of weather, elsewhere in the world, which might affect the UK.
In places like Australia (where an entire continent can experience virtually identical weather conditions), weather forecasts can normally assumed to be reasonably accurate for a week or so ahead. In the UK (where weather conditions can be vastly different only a few miles apart) forecasters can't do much better than forecasting 3 days ahead. The 5 day forecasts, which you find on some websites, are (for the final 2 days) largely based upon historical trends rather than genuine 'forecasting'. The 10 day forecasts, on a few websites, do little more than offer 'guesswork' based upon the same historical trends.
If the proponents of global warming are correct, the trends will be for our summers to become slightly warmer but considerably wetter, with a greatly increased risk of widespread flooding. However that doesn't mean that a particular summer will be either a 'barbecue heaven' or a 'complete washout'. The only way to (almost) guarantee a dry summer is to move away from the UK to somewhere with a generally drier climate.
Incidentally, last June was LESS wet than average, and the number of wet days across the whole summer was roughly the same as in the two previous years:
http://www.metoffice....e/uk/2009/summer.html
Chris