As per Bs 5839-1, spacing between heat detectors is 7.6m how it is almost double 15.2m in nfpa 72?

It depends on the detector's sensitivity as per the manufacturer's guide.

While some detectors have high sensitivity, they could cover an area up to 15m in diameter.

Other detectors with lower sensitivity would cover less area.

It is most important to refer to the manufacturer specifications when designing a fire detection system