A flat tyre is not automatically a roadside repair job. Follow the Highway Code to reach a place of relative safety as soon as safely possible. Do not attempt repairs on a motorway. If you cannot reach a safe place or exit safely, stay belted with hazard lights on and call 999 for police assistance.
Leave from the side away from traffic only when safe and possible, and wait away from the vehicle and moving traffic. Check the ground beyond any barrier before crossing it. Follow the breakdown service’s instructions; an emergency area is not a workshop.