I think the simplest and fastest way is to use special websites in internet which provide weather forecast. But I recommend to use several sources of such information to be sure in this!

The weather in UK is rather rainy, so it is really important to check the weather forecast every day. For this purpose I suggest to use https://www.meteoprog.co.uk/en/ which is a really effective website if you want to be on the wave of todays temperature and changes. it is a really trusted method and really reliable website where you can check everything.

How to check the weather in London?