Hey 🙂 4th year med student at Oxford here! Best advice I'd give you is
a) Make sure you know your content super well (they don't expect you to know about stuff you haven't covered in schools yet but if for example you say in your interview/ personal statement that you enjoyed learning about how vaccines work, make sure you remember how they work! They're not just looking for someone who gets all the answers right, but if you're sharp on the content then you leave a good impression.
b) This is really key- try and get confident in talking through your thoughts and ideas relating to the parts of biology you study. So if they ask you about a biological process (e.g. clotting cascade which you may cover if you do Edexcel A level bio!) then make sure you're comfortable in talking through it. Also practice talking through experimental results (like graphs, or maps showing incidences of diseases etc) and coming up with theories to explain those results. Essentially, you want to make sure you can clearly communicate your thought process, which doesn't mean you can't back-track and change your mind about something you're saying, but it gives the tutors a really good idea about how you approach problems.
c) Hard one, but try smile, be relaxed and friendly. You could spend six years with these interviewers- convince them that you're someone they want to spend six years with.
I never used online tutoring services and apparently (found this out from my director of studies over wine) was among the top 5 at both colleges I interviewed in. That said, I do tutor myself, and have worked with students who found it very helpful. Nothing wrong with it, but equally may be worth saving the money and trying to see if maybe your bio/chem teachers could do a mock interview with you.