I am not from UK either, my school does not predict grades, it's the policy, so my referee said in his reference that the policy of the school is not to predict grades. And the offers I got were based on my actual grades at the moment of application (this is true for all international applications). In UK it's the way school system works, but not so in other countries. In UK tutors will predict grades and the student does not see the reference because it's sent directly to UCAS by a tutor. Not so in our case, we see the reference and have to copy/paste it in the online aplication, so grade prediction is not valid. The important thing to remember is that your application is first read not by the department admissions tutor but by the international admissions tutor, and only then with his/her comments it goes to departments. The international admissions tutor knows school system and grades of your country very well. Actually why don't you email the international admissions tutors of all your chosen universities and ask them the question. It's a good idea anyway to outline your situation and ask if they will consider your application. They might say that your qualifications are not good enough for a particular course, for example, or something else. The truth is that every single international application is very different from another and it's best to ask opinion of the international tutor. You most of the time can get their name on the university website, or email the international admissions office, they are always very helpful. But I personally would not predict grades out of my head if your school's policy is not to predict them.