If you are not from the British Isles, then you will need to understand that our weather forecasting is notoriously difficult, due to our geographic position and prevailing winds.
The Met Office is the root of all knowledge and they publish local forecasts that are starting to become quite accurate over 5 days. Not bad considering the fortune they spend on Supercomputers. Local, regional, mountain forecasts can be found at http://www.metoffice.gov.uk. They also have useful Smartphone Apps too.
I also like http://www.mwis.org.uk/ which is focused on mountain weather, but I wish they had an App as I am on the trail when I need the forecast – duh!
I use www.yr.no in Europe, it works well in countries other than Norway too.
In Scotland, you also need to pay attention to the Midge forecast (no-see-um). This is more important than the weather forecast in the summertime. These wee beasties’ can eat you alive! But don’t worry, they have an App for that too at http://midgeforecast.co.uk/ Only iPhone at the moment 😦
Likewise, tide timetables are useful for coastal walking particularly for planning estuary crossing and avoiding high tides on certain sections see http://www.tidetimes.org.uk/.
Lastly, you can be stopped dead in your tracks due to flooding. The Thames Path, for example, can be impassable in winter so check first on http://www.shoothill.com/FloodMap/ or with the Environment Agency at https://www.gov.uk/check-if-youre-at-risk-of-flooding
Forewarned is forearmed.