Has anyone thought that he might have mixed up meters and feet? I am pretty sure the Hawker works in old money and most of the planes he works on with BA are likely to be in meters. 
All aviation in the UK and internationally works in "old money" although I've flown gliders from Europe with metric instruments, which are confusing. If he flew any types with metric instruments he would have been well used to converting to and from feet as that's what air traffic control work in. A Hunter wouldn't have been fitted with metric instruments.
The Fail have jumped to the conclusion that the pilot was at error, but I can't see anything of that in the AAIB bulletin.
The only altitude reference is from secondary radar, which has an altitude resolution of 100 feet, isn't referenced to QNH of the day and the report mentions the 6 second update rate. A lot can happen in 6 seconds, certainly, a gentle climb from 200 to 500 feet before entering the loop wouldn't have been apparent from that data. Secondary radar is designed to identify aircraft at cruising flight levels. Who knows how accurate it is that close to the ground?
I don't actually believe the entry height of the manoeuvre is relevant. Whilst an aircraft with low wing loading and low or no thrust such as a glider essentially conserves energy through a loop, exiting at close to the entry height and speed if flown properly, an aircraft with a high wing loading such as the Hunter would be very "lossy", relying on thrust throughout the manoeuvre.
The interesting thing would be to know his height and speed at the top of the loop. Here he would decide if he could "pull through" the vertical axis and recover level flight safely above the ground, and he would have the opportunity to roll back level and abort the figure if not.