It's hard to directly compare Bellingham and Foden but La Liga is widely regarded as an easier league and Foden's scored the same number of league goals, in more games, but also outscored Bellingham in the CL. Foden's the better player. But I don't think you have to sacrifice one of them. You sacrifice other things instead. Rice for example, he's better in a box to box role undoubtedly but he's not better than these two so he has to be sacrificed.
I've also got a suspicion we'll see Southgate copy Pep with a CB at LB to cover the space and enable a bit more freedom for Phil even if he is "off the left" and drifting centrally. Many games this season Gvardiol has been pretty much on his own on the left with no option but to pass inside or backwards. But we can still defend the situation if we lose the ball because of everyone else shifting over.