I know a girl at my college who got kicked out of PhB Science last year, at the end of her first year (she's in Arts/Science now). But she was kinda asking for it, considering she was no way near the average both semesters (credit average both times). She failed maths both semesters, despite taking supps for both of the them (she got 48% on the supp for MATH1115... ouch).
One of my good friends got a mid-credit average in his first semester of PhB and a 79 for the next two semesters, and he's been fine with it. And my PhB Arts neighbour has only gotten her average once, but the rest have been close enough (78+, I think) to get away with it.
They're very reluctant to kick people out after their first year, but if you do badly (and I mean less than 75 both semesters) then you'll probably get kicked out.
Mind you, if you don't graduate with first-class honours, then you graduate with a BA or BSc instead of a PhB - PhB depends upon you getting first-class honours. It's a pretty weird scaling system they use to determine which class honours you get, based predominantly on your honours year, and third year subjects. I think. Not sure exactly.