I don't think it works quite that way - I note with the mexico bet the £25 free bet is not returned - and that there is no cash back in your pocket just two free bets to win more money with (so they say). I could be wrong but that's how I read it??
Another thing - bookies are not going to lose - if this was all money back in your pocket then they would be going out of business very quickly. What I think happens is that you win and you get a credit in your account (except perhaps the money you won with your own deposited funds) and then you can bet that later (which most people will lose on) and you ill not be able to do the system as there would be no free money as an existing customer.
Good luck if it does work.