> it's worth noting that the UK has not, in living memory, attempted to subjugate all of Europe under its ruthless mass-murdering rule
I agree with this point but would like to add a tangential one: The UK didn't do that in Europe, but they did it many other places around the world, as did other world powers. Why is murderous oppression acceptable in Rhodesia/Zimbabwe but not in Poland or England?
Amongst people who think it is, there are basically two reasons:
1. Africa is where brown people live.
2. We (in the UK) tend to have a romantic rose-tinted view of how the British Empire treated its colonies. We imagine that our rule was a net positive for the countries we occupied, bringing civilization to primitive tribes. One of the effects of losing the war is that the German people weren't allowed to forgot the atrocities of Nazi Germany. We, on the other hand, were allowed to forget our own atrocities.
Just adding to your excellent points: The words net positive are used to justify almost every attrocity in history, including Hitler, Stalin, Mao ... torture by the US government (not comparable to those three but the same rationale) ... etc. etc. You have to break a few eggs to make an omlette, as they say.
It's a little more complicated than that.
The Conservative party is officially in favour of the EU, although not all of its members are. Whilst it has allowed party members to campaign for leaving the EU, it hasn't allowed them to use parliamentary resources to fund or support their campaign.
"Anti-EU" is also a vague term. There's a difference between "we don't mind the EU existing but don't want to be a member" - a typical UK Euro-sceptic view - and "the EU is great in principal but maybe it needs more skulls on its uniform" - a typical neo-Nazi view.
Whilst nobody is suggesting banning people who campaign for "brexit" - it would be odd considering we're having a referendum on it - it's worth noting that the UK has not, in living memory, attempted to subjugate all of Europe under its ruthless mass-murdering rule. Given their past, German sensibilities may not be as receptive to the idea of wanting to change Germany's position in Europe, and parties which adopt policies of that ilk may be viewed with deep suspicion.