I have a Garmin instinct. I’ve set the Position Format to British Grid (I’m in the UK). In doing this it fixes the Spheroid to “Airy”. I want to set the Spheroid to “WGS 84” whilst keeping British Grid. How do I go about doing this?

I have a Garmin instinct. I’ve set the Position Format to British Grid (I’m in the UK). In doing this it fixes the Spheroid to “Airy”. I want to set the Spheroid to “WGS 84” whilst keeping British Grid. How do I go about doing this?