It is common to climb from approved flypast height to aerobatic height in a fluid motion before commencing the full manoeuvre.
Yes but....
Depending upon the thrust available and power to weight ratio, that climb to the aerobatic entry altitude at 29 degrees pitch up, could be consuming energy which really should be saved for the maneuver. I Am far from a Hunter pilot, but any low performance propeller aerobatic aircraft I have ever flown requires a loop entry from either level flight, or a slight descent, or it will get mushy over the top. If mushy over the top, you're having to surrender altitude to gain airspeed down the back of the loop, or else you may pull too much with the ground rush, and mush semi stalled all the way down the back side of the loop, leveling at a much lower altitude than intended. Mushing down the back of a loop, because you did not accelerate to the flying speed you'll need to pull the required G, will consume a lot of altitude.
I'm confident that these factors are considered during the planning of the performance, I'm hopeful that pilots are entering the planned maneuver with the momentum they need, stored in speed.