Does anyone know of an official (and indipendent!) guide or ranking of Master courses, per University and/or per subject?

The closest thing is the Teaching Quality Assessment score. This is a system which is currently being phased out, but departments still advertise their scores under the old system. It is not an ideal guide as it rates departments and not courses. The RAE score is an assessment of 'Research Quality'. It can also be used as a guide to the standing of the department, but no necessarily the quality of a course. Masters degrees are expensive and hard work - before applying to a particular course ask to be sent testimonials and figures on the employment of previous graduates.