Oxbridge is very much exam based assessment. With your degree depending only on the exams taken at the end of the final year. So the take-home assessments are there just to make sure that you understand what you're doing and can progress to the next part of the course.
I must say that I am rather glad the degree I did at a Scottish university was based mainly of 4th year exams, with an element of 3rd year exams and coursework.
The idea of a averaging all of your work across your course would have been a disaster for me as I did rather poorly in year 1, scraped through year 2 and did spectacularly well in years 3 and 4.
