For the written work and personal statement, I think that you should write about things that you would be comfortable to discuss at the interview. I would suggest not writing too much about texts that you have studied at GCSE/ A-Level as it can be difficult to get your own ideas across and I think it is much easier to write/ talk honestly about texts that you have had less formal instruction on. If there are particular books/ theories/ parts of the university course that you are interested in then include those even if you don't feel 100% confident in what you are saying. As you are writing, try to think of any problems with what you have written and consider how a reasonable person may argue against you. Defend your ideas against these arguments if you can, but don't be so stubborn that you dismiss major problems with your argument or try to cover them up. It is okay to accept that there are very good arguments from the other side, but that on balance you have an opinion that favours one side.
If you are very interested in drama/ poetry/ prose then the written work and personal statement are your chance to (potentially) sway an interviewer into your areas of interest which will make the interview much more enjoyable for you. Personally, I love drama and don't care much for most poetry/ prose other than in the analysis itself, so I focused most of my personal statement/ written work on drama and theory. The questions that the interviewers asked about my personal statement/ written work were difficult, but I enjoyed the conversation and counter-arguments because some of them I had considered and dismissed already and others were more surprising and compelling and forced me to rethink ideas that I had previously been very committed to. It is okay to consider new information and decide whether you want to continue to defend your argument or whether you want to change your mind. You are not applying because you are perfect, you are applying because you are capable and willing to study the subject at a high level.
I would suggest writing simply and honestly about your ideas, making links between texts, and having a clear structure that makes your work easy for someone else to follow. You want to avoid misunderstanding as much as possible as it can make conversation difficult if you use words that you aren't 100% sure of, or if you use the wrong word. If there are any questions you have about literature, then I think it is okay to include them in your written work/ personal statement. I think you should attempt an answer, but you don't have to pretend to be an expert. You just have to be willing to have an opinion and to think carefully about literature.
For the ELAT, I would say to have fun with it (as much as you can) and be confident in your own abilities. Don't treat it like an A-level paper or worry about how it will be marked. It is just another opportunity to demonstrate your skills and to show how well you can construct an argument on unseen material under time pressure. You can write about anything you find interesting, so if you notice anything about form/ language that stands out to you, then write about that. It's okay to not have all the technical language to explain things, but you can still describe what is happening and the effect it has on the texts and compare them that way. Try not to make statements about individual words/ sounds, but look at the text as a whole as well as its individual features and get a sense of what is similar/ different about how language works in the two texts.
If I remember correctly, there were a range of extracts to choose from including non-fiction, prose, poetry, etc. so read through them all and pick what you find easiest/ most interesting to compare/ contrast. Don't choose texts that you find difficult because you think they will be more impressive, just pick two that you understand and compare what you find striking/ interesting about their form and language.
Best of luck with your application!