I think, if you put it into perspective, that the number of flights that are actually subject to "serious" system failures are relatively few - thank God. This is, from the sound of it, one of those few. Modern airliners are electic 'critical' and to have such a major interuption early on in the flight deserves a real close looking at. I applaud their decision to spend time diagnosing and eventually restoring most systems but having done so the "safe" decision, in my opinion, would be to land. At LHR they would have all the engineering support anyone could possibly want. Then try and find out what went wrong. The passenger inconvenience, in this particular case, is irrelevant. After all, we are not in the business of risk!
Finally, to answer the originator, if ALL systems had to be fully serviceable prior to departure then very few aircraft would ever get airborne. Your car, presumably, has nothing wrong with it? This is the purpose of the MEL.
Hope that helps.