BRUSSELS—European Union ambassadors were convening on Christmas Day to start assessing the massive free-trade deal the bloc struck with Britain that should kick off next week when the Brexit process finally comes to an end. After the deal was announced on Thursday, EU nations already showed support for the outcome and it was expected that they would unanimously back the agreement, a prerequisite for its legal approval. Speedily approving the deal is essential, since a transition period during which Britain continued to trade by EU rules despite its Jan. 31 departure from the bloc runs out on New Year’s Day. Without a trade deal, it would have exacerbated chaos at the border where checks on goods will have to be increased since Britain is fully out of the 27-nation bloc. The UK Parliament is expected to approve the deal in the coming days, but the agreement will have to be …