what Mediterranean countries did the USSR make early postwar advances to take advantage of England's incapacity to defend the Mediterranean balance of power?

The Soviet Union made postwar advances to Egypt after the 1956 Israeli-Arab War and the internationally condemned jointly invasion of the Suez Canal by British and French airborne forces aided by Israeli ground forces, an event that greatly angered the Egyptian public opinion and made an approach of the Soviet government to the Egyptian government far easier. As a result, the Soviet Union supplied the Egyptian armed forces with weapons systems of all kinds, tanks, jet aircraft, radar systems, artillery pieces, etc. plus extensive technical assistant by Soviet advisers.

The other Mediterranean country the Soviet union approached to was Syria which also received plenty of Soviet-made war matériel and technical assistance.

In both cases, the British and French support to Israel in the early 1950s drove Egypt and Syria away from Western powers and threw them into the Soviet sphere of influence. It is worth of note that the Soviet Union did not request any payment for the aid supplied and avoided any criticism or interference with the Islamic faith prevalent in both countries.