BT Mobile does not support e-Sim you need to use EE
BT own EE but BT Mobile is a different network with different services. BT Mobile is considered less 'premium' compared to EE which is why it lacks additional services like eSIM and Visual Voicemail.
You can't buy a SIM slot that has space for 2 SIMs as UK/EU and US iPhones lack the hardware to read the additional SIM.
If having the line in your phone is really important you might want to call 150 (or 0800 800 150) and speak to BT customer service who might be able to move you over to EE. I was 6 months into a 24 month contract and they moved me over with no early termination fee. You'll need to have a good payment history with BT and not have an outstanding/past due bill.