The Air Accidents Investigation Branch (AAIB) in Farnborough, southwest of London, is looking at the flight data recorder, which records information from instruments on the plane.
Yesterday AAIB experts downloaded "valid data" from the first black box, the cockpit voice recorder, which should give them hours of the pilots' conversations.
"They have started examining the Flight Data Recorder," a Department for Transport spokesman told AFP today.
"The analysis of any data will be done by an international team led by the Dutch."
The boxes - which are actually orange in colour were delivered to Farnborough by the Dutch Safety Board (OVV), which is leading an international investigation into the crash in which 298 people died, 193 of them Dutch.
Pro-Russian rebels controlling the crash site handed the boxes over to Malaysian officials on Tuesday, following an international outcry over the treatment of the wreckage and the bodies of the victims.
The OVV said yesterday that the data from the cockpit voice recorder data "was successfully downloaded and contained valid data from the flight. The downloaded data have to be further analysed and investigated".
If the second black box also contains "relevant information" then the data from both boxes will be combined, it said.
The OVV is coordinating investigation teams from eight different countries, including Russia.
Western governments say the evidence points to the Boeing 777 plane having been shot down with a missile by pro-Russian separatists.