MANCHESTER, England—Britain warned the European Union on Monday it would trigger safeguard measures in their divorce deal if the bloc failed to agree to changes to smooth trade with Northern Ireland, saying the agreement had “come apart even more quickly than we feared.” Brexit minister David Frost told the ruling Conservative Party’s conference he would present a new set of legal texts to support the government’s earlier proposals for change to the Northern Ireland protocol. Frost did not say when the government would trigger what is known as Article 16—allowing either side to take unilateral action if the protocol is deemed to have a negative impact—but British newspapers said he could move by the end of next month. The EU has said the triggering of Article 16 would be “extremely unhelpful” and it would look at all options in response. “Without an agreed solution soon, we will need to act, …