I didn't go to LSE but I went to another London university that was similar. Yes, it was very difficult. They have much fewer assignments and there's a lot less emphasis on professor-student mentoring, so it can be difficult to know what exactly they want and how to get an A. I'm an A student and I ended up with an A, A-, B+, and a pass (pass/failed the class). In retrospect I should have pass/failed the other class that I got a B+ in so that it wouldn't affect my GPA. Anyways my point is that yes my friends and I who went to various London schools found it difficult to get straight As due to our unfamiliarity with their grading system and the few number of assignments (often just one essay at the end worth your whole grade).
That being said - going abroad was the best decision of my life. I would not trade the experience for any 4.0. It is also a common topic that interviewers ask me about or can relate to. Every time they ask me about it it seems to go over well and I think it shows my adventurous spirit and love of learning.
If you are doing it because you think it will make you more competitive or something, then I think that is unlikely. But if you truly want to go study in London, then I would not be held back by the rigor of the courses. For one it is not more than you can handle, and secondly because that is no way to live life (turning down great opportunities for an application). You can choose lower level courses (1st or 2nd year courses) and pass/fail courses if you need to. I took an archeology course for example (which was actually my hardest class), but it is a fun topic of conversation.
