The comparison between the dropout rates for British and German universities is a topic I have talked about with many collegagues familiar with both systems. The core reason seems to be that what is deemed an acceptable dropout rate in the UK is much, much lower than in Germany.
This then leads to all kinds of different actions and policies that lead to the actual dropout rates being so much lower.
British universites that can afford to be picky are usually very selective, and will only admit students they are confident will do well. On the other hand, for many subjects (eg math, CS), German universities will admit almost anyone with Abitur, and then just sort them out in the first year.
British universities tend to offer quite a bit of support for (weaker) students. How much and the nature varies - most universities couldn't afford the immense attention paid to individual students at Oxbridge, but there is bound to be some effort. I've never heard of comparable attempts at German universities.
Lest I give the impression that I consider the British approach superior, UK universities that cannot afford to be that picky in admissions will just lower the standards to the point where almost everyone they do admit will pass (with whatever support they can offer). German professors tend to have much more latitude to only let those pass they deem worthy.