Shergar: The Racehorse and the IRA 8pm, Channel 4 “Somebody comes up with the idea that they’re going to kidnap a racehorse.” In 1983, Shergar was the most valuable horse in the world, worth about £10m. But with the IRA in need of funds, three men in balaclavas with machine guns stole him and demanded a ransom. The extraordinary story is told by racing insiders including commentator Derek Thompson and journalist Lissa Oliver. Hollie Richardson Britain’s Got Talent: Live Final 7pm, ITV1 It’s time to choose a winner! The Golden Buzzer finalists include dance group LMA, drone artists Celestial and fire juggler Liqei Yang, along with public vote finalists Anastasiia and her dancing collie dog Salsa, the Hawkstone Farmers’ Choir and magician Fabian Fox. HR Casualty 8.25pm, BBC One The miniseries spin-off from the geriatric hospital drama continues. This week, Flynn discovers that Col Bard is something worse than a domineering army officer when a callout leads to two injured soldiers.…