There hasn't really been a weather app specifically designed for hill goers in the UK, but now there definately is and it looks the business!
Five day forecasts for all the main mountain regions; forecast maps with rainfall, cloud and temperature charts; mountain summit forecasts and links to avalanche forecasts and weather station data.
It does cost £3.99, but I'm pretty happy with it so far. Check it out.
It's available on Apple's App Store and Google Play, for Andriod users.