I haven't heard anything of the British coating their own bullets as an insult when fighting against India. The example usually talked about was with the munitions issued to their own (sepoy) Indian troops, one of the causes of India's First War of Independence.

In that case the cartridges for the new Pattern 1853 Enfield Rifles were believed to use a grease containing both pork lard and beef tallow. This was offensive to both Muslim and Hindu troops, especially given that the standard way to use these cartridges involved biting them open, meaning the grease would contact the user's mouth.