Rav Chaim Soloveichik said that Amalek is anyone who wants to destroy us for no reason. It would seem that conversely if he would change his mind he would no longer be Amalek (in fact, after WW2 the Americans and British undertook a massive re-education program in Germany). There is, in fact, a discussion as to whether or not he can convert.
I agree that Corbyn is an antisemite but I don’t know if he actually wants to destroy us. Getting back to the Germans, they have a very serious dilemma. Their birth rate is only 1.5 per woman whereas their life expectancy is a bit over 81. Someone is going to have do the work and pay for the old-age programs. That means that they will have to attract immigrants. On the other hand, they have big problems with absorbing immigrants. This is also a source a violent antisemitism, which is a great embarrassment to them.