LONDON—Prime Minister Boris Johnson has given David Frost, the man who led Britain’s Brexit negotiations, a job in his top team of ministers to advise on relations with the European Union, the prime minister’s office said on Wednesday. The surprise move puts one of Johnson’s most trusted allies, a eurosceptic former diplomat and champion of British sovereignty, in charge of managing a post-Brexit relationship with Brussels that has got off to a rocky start. Frost served as Johnson’s chief Brexit negotiator, both for the initial deal, which took Britain out of the EU on Jan. 31, 2020, and for negotiations on the subsequent trade deal that came into force at the end of last year. The government had previously indicated that Frost would continue to advise the government on matters relating to the EU, but his appointment to Johnson’s cabinet of top ministers was not expected. As a minister rather …