The heat had not finished, it does so when:
when 2 minutes have elapsed after the leading rider has completed the heat (12.1.21)
That being the case 12.1.14 applies:
The Referee shall immediately disqualify any rider who they consider indulges in foul, unfair or dangerous riding.
If, in the opinion of the Referee, such conduct produces an advantage to the rider involved or affects the chances of one or more riders, the Referee may stop the heat and order a re-run.
If any rider is unable to cross the finish line as a result of foul, unfair or dangerous riding on the part of another rider who, inconsequence, has been disqualified, the disadvantaged rider shall be deemed to have finished the heat in the placing held immediately before the foul, unfair or dangerous riding and allowing for any advancement in placing following the
disqualification of the guilty rider.
A similar system shall apply for a rider who, in the opinion of the Referee has deliberately laid down their machine or has left the track in the interests of safety.
So Kemp should have been excluded and Thompson awarded second.