Read my quote and you'll see what of your post I object to. Yes, Germany bombed Britain and Britain bombed Germany - that's not in question, and much of the bombing on both sides was of limited military effectiveness. The British had the dubious honour of developing the 'best' strategy later in WWII, and killed many more people in bombing raids in Germany than v.v., but don't think for a moment that the Germans weren't trying to do the same.
But for you to claim that 'we bombed the rest of Europe to bits' is complete nonsense. Take a look at this list of cities and see how much of the bombing in Europe was British action and how much by other countries: Warsaw, Caen, Wielun, Dublin, Leningrad, Rotterdam, Belgrade, Chisinau, Gibraltar, Helsinki, Sofia, Tallinn, Budapest, Frampol, Zadar, Stalingrad, Frascati, Bucharest. It's nothing like the simplistic assessment you present as fact.
And getting back on the subject, while there was a certain amount of grade-separation after wartime damage in Germany, the vast majority of this work was done as part of normal peacetime capital investment.
In Britain the LSWR recognised the benefits very early on, and there are a number of examples on other British railways, and the same on the Continent. A lot of railways were built to the lowest possible budget though, with flat junctions and poor alignments.
Grade-separation has been happening for over 100 years both here and in the rest of Europe, and when and where it occurs is largely down to how much national governments value and provide financial assistance to their railways.
The Swiss are one of the most pro-rail nations, and you see the benefits travelling around their network. Many conflicts eliminated or in planning with dive-unders, additional platforms, tunnels, new lines, fly-overs, junction rebuildings, underground platforms at existing stations. None of that is as a result of wartime repairs, all is down to how much the Swiss value their railways and are willing to invest. Curiously though, many of their train toilets still open directly on to the track, both solid and liquid matter falling straight through a hole!