LONDON—Prime Minister Boris Johnson urged European Commission President Ursula von der Leyen on Thursday to consider seriously Britain’s proposals to change what he called the “unsustainable” way a Brexit deal is governing trade with Northern Ireland. Since it completed its exit from the EU at the end of last year, Britain’s ties with the bloc have reached new lows, with both sides accusing each other of acting in bad faith over an agreement for post-Brexit trade with Northern Ireland. London accuses Brussels of being too purist, or legalistic, in interpreting what the deal means for some goods moving from Britain to its province of Northern Ireland. The EU says it is adhering to the deal, which Johnson signed just last year. Britain proposed on Wednesday to renegotiate parts of the Northern Ireland protocol that govern the movement of goods such as chilled meats, and to dispense with EU oversight of …