Highway Code Rule 274
If your vehicle breaks down, think first of all other road users and
- get your vehicle off the road if possible
- warn other traffic by using your hazard warning lights if your vehicle is causing an obstruction
- help other road users see you by wearing light-coloured or fluorescent clothing in daylight and reflective clothing at night or in poor visibility
- put a warning triangle on the road at least 45 metres (147 feet) behind your broken-down vehicle on the same side of the road, or use other permitted warning devices if you have them. Always take great care when placing or retrieving them, but never use them on motorways
- if possible, keep your sidelights on if it is dark or visibility is poor
- do not stand (or let anybody else stand) between your vehicle and oncoming traffic
- at night or in poor visibility do not stand where you will prevent other road users seeing your lights