Are predicted grades the most important factor when applying to uni?

As above - it depends on subject and University.
You need to ensure that your PS, GCSEs and A level predictions are ALL the best you can make them - as you are not going to know what they look at, so don't cut any corners.

It really does depend on the university and course youre looking at, i would say yes its the most important- BUT if there are mitigating circumstances these will most likely be considered. if your grades are lacking, best thkng to do is contact the uni directly and ask- if theyre v vague then try attending an online open day and speak to one of the admissions tutors for your course directly. good luck

thank for your reply, my grades are slightly lacking (by one grade) but my PS and references are good and my contextual data goes in my favour. so i can hope for an offer

Oh right well one grade is definitely worth looking at then. It still depends, but having 1 or 2 aspirational choices is worth considering- if contextual data is in your favour then some universities actually have lower requirements stated depending on what that data is (first example that comes to mind is Bristol's Contextual Offer Scheme). But as before- double check with unis directly before sending an application if you have doubts.