A useful sequence is to begin with explanation that bridges school chemistry and university reasoning. Why Chemical Reactions Happen is useful because it is a strong bridge text for applicants who want to move from A-level Chemistry to university-level explanation. Principles of Chemical Science is useful because a free course for testing whether the applicant enjoys longer-form university-style chemistry.
For problem solving, use sources that force you to apply familiar ideas in unfamiliar forms. UK Chemistry Olympiad past papers is useful because the most directly useful practice source for unfamiliar chemical problem solving. C3L6 resources and downloads is useful because good Year 12/lower-sixth extension material for students preparing before the UCAS cycle intensifies.
For ongoing curiosity, choose resources that connect chemical principles to demonstrations, research and industry. Periodic Videos is useful because a strong curiosity-builder if applicants follow demonstrations with rigorous explanation. Chemistry World Podcast is useful because useful for showing engagement with chemistry as a living research and industry field. Reactions is useful because accessible explanations of chemistry in everyday and contemporary contexts.
Research experience can help, but only if you can explain the question, method, evidence and limitations. Nuffield Research Placements is useful because a competitive route into real STEM research experience, especially valuable when linked to reflection rather than name-dropping.