Anglo- Germanic alliances have always been something really tricky and not so straight forward. For example you mention this in the 30 years war and the 7 years war they were allies. But they were not always allies even then. Before the 7 years war for example, Prussia and France were allies and Austria and the British were allies. But do to numerous factors these alliances shifted to have France and Austria V Prussia and the British.

But there has been a couple of reasons throughout history, especially the 19th century where Prussia/ Germany and the British were pitted against one another.

Napoleon formed the confederation of the Rhine and this slowly started to consolidate the German states with a Common Germanic identity. One thing that always worked in the French and British, and honestly everyone else’s favor was that Germany was divided. But after Napoleon this common identity only started to grow along side the rise in nationalism.

Fast forward to how Bismarck was DAF ( Denmark, Austria, France, a neat way to remember the German wars of unification ) then the British have a problem. That problem is that they have a new great power in Europe. This is not to worrying at first. But then Germany starts to spread out, colonize and make a colonial empire, as you may guess the British don’t want any more rivals. Not only that. But Kaiser Wilhelm is notably anti British and notably was in support of the Boers in the Boer wars ( only out spoken ) combined with things such as the Kaiser apparently being very happy when there was a chance of war between Britain and France. Germany soon became Europes strongest mainland nation. All of these combined made both populations somewhat hostile to each other.

But what really did it was the Anglo-German arms race. Germany wanted to become a maritime power, partly to protectors trade and to enforce itself in an event of a war. The British were taken back as they’d have always been a maritime power. Now with Germany focusing on a navy things were getting scary for them, not at first though and only when Germany was getting years into it. Germany building its fleet wasn’t entirely focused on scaring the British, they just wanted to build up, but it had that affect. By 1908, Germany and the British both had ordered the new but very powerful dreadnought ships which the British having the first one in the world. This combined with Austria- Hungary and the Italians ordering 5 dreadnoughts between them really made the British nervous. They ordered 11 more dreadnoughts, made an alliance with France to balance it out, and the British wanted to make an agreement with the Germans. The Germans demanded an alliance with the British and then they’d talk about naval disarmament. By 1914, the arms race came to an end with 29 British dreadnoughts to Germany’s 17

Along with this came a lot of small things like the Germans siding with the Belgians after the US and the British were outraged over the treatment of the people in the Belgian Congo. Among other minor things

In summery. The 30 years war, the 7 years and the Napoleonic wars all saw the British and the Prussians form alliances with one another. But when the German empire came around and started its colonization and naval processes, this made the possibility of a German/ British alliance very slim. That being said they were not total enemies either. Remember, in 1914 the British were originally neutral, but the Germans went though neutral Belgium bringing them on Frances, Russia’s and the allies side.