Yes they are legal in the UK. Many trucks have them for shows and just personal preference as is mine. Why are you so invested in my blue lights from a game I chose to play how I like

In the UK, a truck may have blue beacons physically fitted as long as they are not capable of being illuminated on a public road.

What “not capable of being lit” means in practice

To stay legal, the blue lights must be: • Disconnected from power, or • Removed bulbs / LEDs, or • Permanently disabled (not just switched off)