The golden rule of Oxbridge admissions: DON'T PAY FOR HELP. The admissions process is designed in a way that makes that sort of thing unnecessary!
However, it is a very good idea to familiarise yourself with the admission and interview process as much as you can, but you can easily do this for free. The Student Room should be your first port of call, as well as the advice the universities themselves give out, like
here or
here. For more info on interviews, check out
this website. If you really feel you need more help, you could take a look at the free section of
this website; again, I recommend you NOT to pay for any of their services, but I actually thought the free ebook was slightly helpful