there are some british people that can help you for proof reading, but they need to know what's your major and how many words in your dissertation, and your deadline of handin', and they need to know when you probably sent your items within the reference to them and when it is supposed to back to you as well, because you should know the mistakes you did in your paper, in which they not only correct your grammer and speeling but some of the advice that will be included. if there are probably some that you are not sure in your paper after the first correction, you could return to them checking again without any more charges. this is the whole service from them at all.
considering the service price you asked, it could be accepted probably and it could be negotiated with you as well if you need their service indeed.