PN will be able to supply more of the weaponeering information, but as far as I'm aware, the bombs used were basic dumb bombs rather than retarded bombs.
Hence the optimum delivery altitude for effect would require a pop-up manoeuvre.
The pervading thought in those days seemed to be that any radar guided AAA could be successfully jammed by the Westinghouse pod. But the airships had little knowledge of the capabilities of radar-directed AA guns, having withdrawn any such weapons from the UK's aerodrome defences years earlier.
Another reason for the pop-up was the time needed for all those old black dustbins to settle down and compute the release point accurately. You have to remember that nothing other than low level lay down nuclear delivery had been practised for over a decade by the Vulcan force.
Unfortunately, unlike SAC, the V-force in those days did not have a real strategic non-nuclear role - so everything had to be sorted out from half-remembered procedures of years earlier, plus a fair bit of 'TLAR'....
As for fuel burn, very little was known of the additional burn when in formation for such long periods - AAR had disappeared from the V-force nearly 20 years earlier. But one hears that 'Caligula', a staff officer of not much known ability, used the low-weight peacetime training fuel burn figures as he was too stupid to know how to use the performance ODM. Hence on at least one occasion, 21 x 1000 lb ended up in the ocean when the actual fuel burn was discovered to be much higher than his figures.....
I understand he then was then on the receiving end of a 'conversation' with the AOC of epic proportions. No-one had any sympathy for him though.