...to modernise (obsolescence) and upgrade computing power and radar processing to increase performance and capability ... foresight the UK was sadly lacking with the E-3D.
To be fair to those involved, this was all recognised at the time. But given the RAF had just embarked on a policy to run down the process which manages such things, and MoD was in the throes of successive moratoria which effectively froze many programmes for up to five years, there was no way the Gods would admit such things were necessary, or even possible.
As for modifying UK variants, this is often necessary for at least two reasons (although the degree varies depending on role of aircraft). First, the Home Office dictates use of certain kit. This is mandated upon MoD. Second, political industrial strategy. For example, certain technologies are (or were) required to be indigenous. In my day, one was acoustic processing in Nimrod MR and Sea King. It mattered not a jot that CD(C)'s devices ran rings round Marconi's.