There's nothing illegitimate about wanting tips on how to make the best possible application and doing the best possible interview if it comes to it. Schools with a track record of success at Oxbridge often provide this sort of guidance as standard and while it can go wrong sometimes, it's often really helpful. If your school doesn't have the resources or the experience to help you then I personally don't blame anyone for looking for an external company to help. (Getting a good one so that you don't waste your money is probably tricky though.)
Nobody needs an external company to write a PS but yes it's worth looking around for tips on what to include (extra-curricular stuff, something sincere about what you enjoy about the subject) and making sure your spelling and grammar are ok. Not least because other unis, who do place some weight on the PS sometimes, will read it too.
Nobody can give you detailed information about the interviews - there are strict rules about not divulging specific questions and most interviewees take that quite seriously, as they should - but some practice, as pp said, in how the process works - eg for maths and engineering, how to talk through your problem-solving processes while addressing a problem - may well be helpful and again an entirely legitimate thing to do. Again, at lots of schools this will be happening as standard - it does at my sons' school. It's one of those hidden bits of the process that benefits kids at selective/good schools. Nothing shameful about trying to get a similar bit of prep by paying for it - if only because it will make the actual interview feel a bit less scary and weird.
All that said, and specific interview practice with someone good aside, most of the information that can help you is out there on the web and freely available. My heart goes out to the family who spent £6k - there are some real grifters out there exploiting applicants and their families.
The guy who makes lots of good videos about applications is at Jesus in Oxford I think. There is definitely a guy at Jesus who has good and useful videos up anyway. Plus have a search for your subject + interview tips on google and YouTube - DS2 found a video of some students in his subject talking (carefully) about what their interviews had been like and it was really reassuring and quite informative.