It's not a linear system where the goal is to get into the highest rated school possible. Not only that, rankings fluctuate.

QS has HEC ranked #1.

Bloomberg has IMD ranked #1.

You jumped from INSEAD and LBS to Oxbridge because you'd heard of them, but for Europe there are schools all over Western Europe that are higher ranked and more useful, depending where (location and field) you want to work.

If you want to get an MBA and then to work in Europe, I don't see why a European MBA wouldn't be your first choice. Clearly American programs are top-notch and they'll carry weight in most developed countries, but it can't surprise you that European schools feed into European MBB.