UK Met Office weather forecasts - rubbish?

We spend millions on new super-computers yet our Met Office seems incapable of making a decent weather forecast! We planned to meet our son and granddaughter at our local  farm shop in the (promised) sunshine so our granddaughter could enjoy the slides, swings and toy tractors. Sadly the forecast is now for light rain! Honestly, even over a few days the forecasts change so much that they cannot be relied on. Basically a piece of wet seaweed would be as accurate.