So I decided to contact the company (based also in UK) and I got a reply in few minutes
Considering the fact that the esim costs only 10$ and it is valid for 90 days then I decided to give it a go.Dear Valued Customer,
Thanks for reaching us. You can buy our Esim and try it in your watch. Hopefully, it will work. If it doesn’t work we’ll refund you.
The ESIM Support Team
I opened the Mobvoi app on the phone, then Scan QR Code and select Other as Telecom operator and Other as country (as it is global operator). I then followed on the watch and downloaded the carrier data successfully.
I restarted the watch and enabled the Mobile Data and enabled roaming (as the carrier is global) with my surprise, I can make calls, SMS and use data. I cannot share the phone number with my phone so I have a different one for my watch. I have set up the preferred network so I can have the best tariff. I am very happy as I can now go without the phone in situations like running, swimming in the pool, or have a backup for my phone.
As I mentioned the provider offers a global esim so anyone can buy it (with also 5G support) with the exception of Italy, Egypt, Benin, Ukraine, Somalia, Syria and Maldives (in Fact in Italy there is a more expensive option at the moment).
If you need more information please let me know. I will testing during the week.