Air-frame heat is definitely an issue, but it's solvable. According to a BBC documentary that's on youtube, the Skylon design uses the same concept as the SR-71

Air-frame heat is definitely an issue, but it's solvable. According to a BBC documentary that's on youtube, the Skylon design uses the same concept as the SR-71, which is a corrugated skin that allows a great deal of thermal expansion and limits conduction of heat from the skin to the air-frame. They also apparently have a novel composite material, which could be a big advantage since machining titanium alloys like the SR71 used is a serious pain in the ass.