The ASI and Altimeter were tested on the bench and were found to be working fairly accurately. However, the aircraft system couldn't be tested due to damage nor could they be seen in any footage to confirm that they were fully operational either. So it is an assumption that they were and the pilot either ignored them, mis-read them or carried on regardless, therefore, only one of these could therefore be fact but which one.
I agree there are chunks missing from the AAIB report, such as due to the seemingly poor maintenance standards (out of date cartridges, MPD's not complied with, part no incorrect, faulty G counter and unknown servicing schedules) in the last 6 years that have been highlighted by the report. It seems odd that the CAA have not given more of an in depth report on the standard of the rest aircraft frame. After all the seat cartridges being out of date are irrelevant to why, as is the compliance of the MPD if this wasn't the why. I know they are solely to recommend improvements for safety but an aircraft poorly maintained or not to standard is a safety risk. They seem to have gone through the pilots info with a fine toothcomb and his log books but not so much on the aircraft and its books.