Writing a book has little bearing on a degree in literary analysis, which the ELL course at Oxford is. You could mention it briefly in your PS but I would not spend more than 1 sentence max discussing it as it's simply not relevant. Again, the degree is in literary analysis/criticism, not creative writing. Don't confuse the two.
Also be aware the "language" part of the "English Language and Literature" title refers to English philology i.e. the study of Old/Middle English and (potentially, depending on options you take/which version of the course you choose for the final honour school), English historical linguistics. It does not mean "language" in the sense of GCSE English Language (which is a kind of vague all encompassing subject for any kind of English composition and creative writing) nor the A-level English Language (which carries on this from the GCSE although includes a bit of basic linguistics content).