Does Apple watch 8 cellular bought from India work in the UK?I

There are only 4 providers that will offer Watch cellular eSIM support, EE, O2, Voda, and Three so you would need to ask them.

The official blurb is...

Apple watch in another country. Watch 8,  SE 2, UltraTo cover the LTE and UMTS bands used around the world, there are two separate cellular models for Apple Watch Series 8, Apple Watch SE and Apple Watch Ultra. They have been optimised for use in the country of purchase, and not all models work in all countries or regions. There is no single model that supports cellular service worldwide. To see which network providers in your country or region offer cellular service for Apple Watch, refer to the chart below. 

Apple Watch - Carriers - Apple

Main thing is No Apple Support and No Warranty for Watches bought outside your home country

There are 2 cellular radios

• Canada, U.S., Mexico, Puerto Rico.
• The rest of the world.

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