With regard to Aylesbury, until 1960 it was served by London Transport Metropolitan Line trains from the City; these were hauled by electric locos as far as Rickmansworth, then steam for the rest of the journey. However, electrification of the line was only extended as far north as Amersham, and this became the terminating point for London Transport services (formed of A60 stock); therefore, is it not reasonable to assume that - at that time - it was deemed that there was insufficient demand to justify through electric services from the City to Aylesbury, and that this town could be best served by DMU services from Marylebone?
On the general question regarding electrification schemes in the east of England, it really is necessary to see the BR investment submissions to see why lines were electrified in the sequence that they were, why some routes were singled (such as north of Ely to King's Lynn), etc; does anyone know if these are available for analysis in either the NRM or at The National Archives in Kew?
Government specified BR's external financing limit, the amount that could be spent on capital investment (and when it could be spent), the required rate of return on investment programmes, etc; there were procedures laid down for investment formulation/analysis/evaluation, and BR had teams of competent managers who knew how to "play" the systems to maximise benefits and minimize capital expenditure (which generally only related to the "betterment" element of a project's costs). A direct consequence of this was that sometimes desirables were missed from schemes; an example of this which comes to mind is the electrification of just the slow lines in the Northfield/Longbridge area.
BR was, of course, a vertically integrated railway, and this enabled it to take a much more holistic approach that is possible with today's fragmented structure; moreover, the creation of the business sectors in the early 1980s enabled it to look at matters much better than had been possible under the previous regional structure. This, together with a government change which allowed BR to cram more people into trains than had previously been allowed, perhaps accounts for some of the electrification schemes in the east of England.
Certainly, my understanding is that the class 315s were originally procured just as Shenfield replacements, and if you look at the fleet size roundly 30 8-car trains replaced 30 9-car trains (formed of class 306 EMUs); but the change to the "cram-'em-in" factor meant that there was a surplus of EMUs in the NSE area, and in some places it was cheaper to string up wires than replace first-generation DMUs. Because only the "betterment" element of the cost had to be justified, the starting point for a project would be "how much will it cost to replace the existing DMUs with new ones?" Using spare EMUs was the basis for electrification of the Southminster branch and Romford - Upminster in the east of England (and also to East Grinstead).
Reading 'Informed Sources' articles in old copies of 'Modern Railways' might help understand some of the decisions made years ago which seem "strange" when looking back with the benefit of hindsight, but Roger Ford wouldn't necessarily have known all that went on behind the scenes; just electrifying as far as Royston would have made sense to the BR managers responsible for developing the GN Suburban Scheme, and it enabled them to get that project through the investment approval process. Going the final few miles to Cambridge (or even going further north up the ECML to Peterborough) seems to have been a major omission when looking back, but that may have "killed" the GN Suburban Project completely; getting wires as far as Royston provided a platform for further enhancements at a later date, and no doubt the wiring of the ECML through the multi-tracked sections of North London would have helped justifying investment in the electrification of the ECML through to Edinburgh.
And, in turn, didn't the electrification of the ECML enable Royston - Shepreth Branch Junction to be electrified on the cheap? Wasn't some equipment made redundant by strengthening the power supply (or something!) on the Hertford Loop re-used to extend electrification north from Royston?
Or have I imagined it!