iPhone 12 dual sim capability

iPhone XS, iPhone XS Max, iPhone XR, and later feature Dual SIM with a nano-SIM and an eSIM.1 An eSIM is a digital SIM that allows you to activate a cellular plan from your carrier without having to use a physical nano-SIM.

eSIM on iPhone is not offered in China mainland. In Hong Kong and Macao, iPhone 12 mini, iPhone SE (2nd generation), and iPhone XS feature eSIM. Learn about using Dual SIM with two nano-SIM cards in China mainland, Hong Kong, and Macao.

To use two different carriers, your iPhone must be unlocked. Otherwise, both plans must be from the same carrier. If a CDMA carrier provides your first SIM, your second SIM won't support CDMA. Contact your carrier for more information. 

If you have an enterprise or corporate cellular service plan, check with your company administrator to see if they support this feature.

The iPhone 12 has 1 nano physical sim and one eSim. See here: https://www.apple.com/iphone-12/specs/

According to this list: https://support.apple.com/en-us/HT204040

It looks like EE and Vodafone support the eSIM in the UK.

The iPhone 12 supports one physical SIM and one eSIM.

The exception is devices sold in China, Hong Kong, and Macau, which do not support the eSIM, but instead support 2 physical SIM cards.