The most important and first things to do are to ensure your own safety, check for any injuries, see medical attention as soon as you can, and to contact the police (if you’re able to, if not, shout or wave to passers-by to get help) via 999. Thereafter, ​​collect as much information as possible about the incident.
This includes noting the license plate number, make and model of the vehicle, and any distinguishing features. If there are any witnesses, ask for their contact details. After which, contact your insurance provider to inform them about the incident, and provide them with all the relevant information, including the police report number, so as to get guidance on your claims process. Lastly, consult a lawyer and get legal advice if there are any injuries and property damages involved.