We all know that BT does not yet support eSIM and Apple watch cellular, but the question that has not been answered before is when will BT add this functionality to BT mobile? Quote from btplc.com: Working with EE, we’ve driven the adoption of eSIM (electronic SIM) technology. Instead of being a removable card, eSIM sits on a chip inside a device – which lets smaller gadgets like watches connect to a mobile network. This technology was instrumental in the launch of the Apple Watch Series 3 on EE’s 4G network. The watch lets wearers leave their phone at home and still be connected.
BT can do this. When will they do this on BT mobile?
My guess is that it won't be anytime soon. As BT is a MVNO there may be technical reasons, or even contractual reasons why it isn't available.
The main networks like to keep their 'premium' status in the market by allowing their MVNO's to have fewer services such as wifi calling, and when they are made available it is some significant time later. The fact that BT own EE is somewhat irrelevant as they would be competing for different segments of the market.
Being cynical, BT Group would probable prefer to only offer it on EE, knowing that those Apple customers keen to have the latest tech are likely to move to EE, increasing their revenue.