deefer dog's quote of mine from an earlier post was written from memory and I have since re-read the Aircrew Manual, including the section that clivewatson quoted in his post and, yes, I was wrong and have re-learned something. The folk lore was that it was fully controllable if below 0.9M but at high IAS it obviously isn't. However, the Aircrew Manual gives no clue as to the IAS at which this occurs and I have never heard of anyone having this problem other than when exceeding 0.9M.
With respect to the apparent reduction in pitch rate passing the down vertical, this could have been caused by a visual illusion when viewing a loop head on because the eye perceives pitch rate as a function of rate of change in apparent fuselage length, and this is a function of sine pitch attitude and therefore non-linear (I have posted this point on another thread already).
The 425 kt loop entry speed in the Aircrew Manual is way in excess of what is needed for an aircraft in a low level display and was typical of the margin applied for inexperienced pilots in recommended Pilot's Notes speeds from documents of that period. The selective use of flaps during a display is quite usual and the display teams such as the Black Arrows and Patrouille Suisse used flap to generate more drag to give better throttle control over the top of loops. I always respect the 300 KIAS limit for the flap; I cannot comment on what others do.