55% was the grade boundary for an A in the 2019 A levels, largely because the paper was quite different to what people were expecting so it caught a lot of people off guard. I imagine the grade boundaries will be higher now that students have more of an idea of what to expect, but 80% for a B is still quite high! I think my school used 90% A*, 80% A, 70% B etc., and that worked out pretty indicative of how people did in the real thing (although we were the 2019 cohort, so that was based on old spec past papers rather than ones for the current spec).