Changeable weather and capsizing are the most common issues when sailing in the UK, so always wear a buoyancy aid or lifejacket (whichever is most relevant to the craft you are sailing) and carry waterproofs and extra clothing. Always get an up-to-date, local, marine weather forecast.
If you are sailing in an area with few other boats, consider taking some means of communication in case of difficulty. Handheld VHFs or mobile phones (in a waterproof case) are useful, depending on location. The RYA Day Sailing course will teach you all you need to know about going on a short journey. Once you have gained more experience, you can start to plan your own day sail.