So much buzz about "black boxes" in media... I do not believe that UK investigators will hear the crew saying that they see Dr. Evil (or Mr. Putin) in a fighter shooting at 777. They will likely see/hear nothing.
Studying debris seems more interesting to make sure what elements actually hit the plane. This could at least prove that it was (or was not) BUK. If not (i.e. no BUK shrapnel found), then the story would begin anew.
There were some discussion here about A2A R-60. This looks very unlikely. The missile is too small and, more important, having a passive IR seeker, it would be heading toward one of the engines. And even if coming from the front it would explode some 5-10 m near the engine, i.e. well away from the 777 cockpit. Then, if the engine were hit, it should not be a fatal damage to the aircraft. Known cases when R-60 was used show that even a (much smaller than 777) planes (F-15 and Israeli Kfir) were able to continue flying, at least did not fall apart. Continuing on the A2A "option", R-27 (by the way, Ukraine manufactured at the "Artyom" plant) is much more serious thing, with semi-active radar-based guidance and a 40 kilo warhead (i.e. 12 times as much as R-60).