Our experience (DS final year) is as described by Ginfordinner.
Christmas and summer exams treated the same - first year, 40% to pass and any failures re taken in August. All needed to be passed to progress to Second year and none were mocks.
In 2nd year, where all marks counted towards degree, Christmas and summer exams all counted and any failures were re-taken in August. However, re-takes are capped at 40%, so could massively affect final degree grade if a number of re-takes are required in second year.
As gin said, good study practices now will reap benefits in future years. We know a number who have stumbled in second year, and had to retake in August or, in one case, the whole year.
DS was never top of class (ABB A levels, doing a science at a RG) but very organised with revision and very hard working, he is currently on for a 1st and will be disappointed if he misses it, all his own pressure, not ours.