That’s the trouble with long engagements: sooner or later, someone prettier is bound to come along, and Telefonica de Espana SA has been engaged to Unisource NV since telecommunications time immemorial. That someone prettier is of course the Concert alliance between the soon-to-be-one British Telecommunications Plc and MCI Communications Corp, which is so much further down the track than its rivals that it has the most comprehensive skills base in place and can deliver more captive destinations than any of its rivals. It in turn desires Telefonica because it believes that the Spaniard and MCI can together deliver most of the best bits in emerging Latin America. Under the agreement announced Friday, British Telecom will take a 2% stake in Telefonica, and the Spaniard will buy a reciprocal 1% stake balanced so no money need change hands. In Latin America, Telefonica International SA and MCI will also create a pan- American joint venture, Telefonica Panamericana-MCI, with each partner holding 50%. Telefonica Internacional already operates communications companies in Argentina, Brazil, Chile, Peru and Puerto Rico and had holdings in the US and elsewhere in Latin America, MCI exulted. As in those interminable medieval wars, where nations were represented by galumphing knights as national champions, telecommunications companies regularly change sides and shift alliances, and so British Telecom will have to bid farewell to its banking friends in the Airtel consortium in Spain.