I think it is unethical, for several reasons.
These people operate a business a large part of which revolves around asking students out of the blue if they would provide fairly exclusive information for free (and don't start about 'winning an ipod' The very language suggests it is a gift [read:bribe], not a payment). Basically, they are leeches. And the business they operate is lucrative, make no mistake. I think I remember, for the full week hardcore private training, it costs parents several thousand pounds.
Another thing I don't like. They are playing on people's insecurities. The student's fear that they might not be good enough without help. They play up the myth that Oxbridge is some 'promised land' and that in order to enter, you must be special. To a certain extent, they are exploiting people.
Finally, the last thing I do not like is the fact that (according to their website) they actually work. Suggesting that the whole delicate imprecise interview system can be manipulated if you have enough money; also destroying any notions that entrance to Oxbridge is purely meritocratic.
As a final muse, think about this; if every student took their training courses, they would become redundent. Their whole business model revolves around the fact that some students parents will not, for whatever reason, shell out the money.