The BMW British Unmarked Pack is a mod for Grand Theft Auto V that includes several British police vehicles in unmarked versions, specifically BMW models. These models typically feature a sleek, unassuming exterior that blends easily into traffic, common to British unmarked police cars. Furthermore, the mod would have integrated siren and signal systems covertly installed within the vehicle, which remain off until activated during a pursuit or an emergency.

The pack might encompass a range of BMW models such as the 3 Series, 5 Series, and the X5, to provide an authentic representation of the actual unmarked police car fleet. As this mod pack is based on high-performance vehicles, the cars would likely showcase excellent acceleration, high top speed, superior handling, and the ability to withstand damage. Details such as police liveries, interior details, and British-style emergency equipment that further add to the immersive gaming experience could also be included. 

It should be noted, in line with being unmarked, these vehicles won't exhibit the typical, easily identifiable police car characteristics at face value. All the emergency functionalities and police paraphernalia are hidden, activating only during the necessary game scenarios.

Credits:
World MC & Britishgamer88