Done quite a few dissertations as part of my LLB and thankfully done pretty well with them. Ended up doing three in my final year (one standalone and two as part of two of the modules I had chosen).
In terms of picking a subject and question, firstly I would echo the age-old advice of choosing something you have at least some type of interest in. This is because with LLB dissertations (given the amount of research and time you have to devote in order to get a decent grade) if you have no interest in the topic it makes it a lot more difficult to carry out effective research and maintain attention to detail imo.
Personally, in choosing my subjects I tried to focus on what were "hot topics" at the time as this meant there was a lot more material to research from. To give some examples, this included the debate re privacy law, the ban on short-selling and the assisted suicide debate. However one caveat if you choose to do this is that you will have to ensure your dissertation has something to differentiate it as loads of students will choose to write about "hot topics" so you won't want to bore the reader.
In terms of the actual research itself, all the usual suspects: journals, textbooks, Westlaw/Lexis etc. What I would advise though, is that because of the sheer breadth of material you will access to, choose a list of about 25-30 sources and focus on those. Of course this number could go up or down depending on what the word limit of your dissertations is (mine were 10,000 words for one and 7000 for the other two).
Also try to think outside the box if you can. For example I looked through Hansard to see which MoP had discussed/debated the subjects I was writing dissertations on and contacted them to get their opinions.
Lastly, in terms of how to phrase the actual question itself, this shouldn't be too much of a worry. Once you have decided what the dissertation will be about you can amend the title accordingly so that it addresses the focus of your writing.
Best of luck.
