I must add the same caveat as fogbound, working for BCFT (I teach groundschool in fact).
You say slug that you have paid a non-refundable deposit. Since then they have changed the conditions. In that case whatever it says on the contract I suspect you can get your money back if you do wih to change. It might be an idea to at least threaten it even if you decide to continue with Oxford - you might grab their attention, and they may start to treat you like a client.
If you are not satisfied act like a good consumer - go elsewhere. There is no magical "Oxford dust" that makes you pass exams. That takes hard work. Oxford groundschool is good but so are most of the groundschools in the UK as far as I can ascertain, having given private tuition to struggling students who have sat courses in most of them. Of course we are the best
