Just to mention this again, because it doesn't seem very clear. DHK can not operate the way they do now after a Brexit. We can go from and to the EU from EMA, but we cannot operate within the EU anymore. So making EMA the main base is more or less senseless.
Leipzig is and will be the main base, so there will be a solution and I think it could be a new AOC. The UK AOC is not needed, as flights to the UK can be operated from the EU to EMA etc by any operator, no need to keep DHK the way it is. EMA hub of course will stay, because it doesn't mean that we not gonna send parcels to the UK anymore. Interesting will be what will happen to the pilots? EU pilots, could just move over to the new AOC, British pilots, due to lack of working permit in the EU can not!
More likely they would expand the UK base and make use of the lower tax rates.
What ever tax you are talking about, being a german company, DHL has to pay taxes in germany anyway, no matter of their base.