I'm looking for an online data set which can provide present UK-wide average temperatures, rain likelihoods, humidity levels and other meteorological data.
Is anyone aware of an API/flat table data source that can provide this information?
A major resource for grabbing live weather data is through the OpenWeatherMap API.
The free plan offers up to 60 calls per second and access to present and 5-day forecast for individual locations, as well as weather map layers (temperature, precipitation, etc.) that can be integrated with Leaflet, OpenLayers and other WMS systems for regional representations. For personal use, the free plan should be sufficient for your needs.