Hey there and welcome to the Samsung UK and EU Community Forum
There are two ways to turn off mobile data whilst you are in India....
Settings
Connections
Mobile Networks
Data Roaming
Slide Data Roaming to OFF
Settings
Connections
Data Usage
Mobile Data
Slide Mobile Data to OFF
In terms of WiFi calling, pull down from the top of your screen to open the Quick Access Menu and find the WiFi calling icon, make sure it is highlighted.
NOTE - You may need to ask your UK Network to provision WiFi calling.
S24 Ultra