Know the local laws, street signs, and speed limit. Practice in a safe beginner space. After you are familiar with the different visibility and turning radiuses required when making turns whilst driving on the left side of the road, you can start driving on busier roads.
You should always carry a valid drivers licence and proof of vehicle insurance when you drive. It is a criminal offense to refuse or give false details which will result in arrest. Unlike in the USA and maybe other countries, the UK police do not need permission or a reason to stop and search you, your belongings (handbag, rucksack or pockets) or your vehicle if stopped while driving. Do not attempt to stop them as you will be detained or arrested.
In the UK, the left hand lane is considered the “slow lane” and some drivers may pull over without checking properly which can cause an accident. Be aware that using the hard shoulder of a motorway to undertake vehicles will also result in a possible arrest.
If driving a friend’s vehicle always get their permission and check your own insurance policy to make sure you are permitted to drive other vehicles.