Muncrun I wouldn't say it was "extremely difficult" to get a tenancy...but it is extremely difficult to get a TOP tenancy. Although yes, you will usually need a 2.1 // 1ii, (unless there is some reason for you obtaining a 2.2, which was beyond your control).
For the top Chancery and Commercial chambers you would really need a 1st or at the very minimum a high 2.1 along with mooting/debating experience and ideally some prizes / scholarship.
For example, look at the CV of this new recruit at Lord Grabiner QC's chambers:
"EDUCATION
BA History, Downing College, Cambridge (1st class)
Post Graduate Diploma in Law, City University (Commendation)
Bar Vocational Course, Inns of Court School of Law (Outstanding. Top of year).
OTHER ACADEMIC ACHIEVEMENTS
Baron Dr Ver Heyden de Lancey Prize (Middle Temple, 2003)
Du Cann Memorial Prize for Advocacy (3 Raymond Buildings, 2002)
Scarman Scholarship (Inns of Court School of Law, 2002)
Sibel Dedazde Pro Bono Award (Inns of Court School of Law, 2002)
Queen Mother Scholarship(Middle Temple, 2001)
Queen Mother Scholarship (Middle Temple, 2000)
Piley Scholarship (Downing College, 1999)
Buchanan Scholarship (Cambridge University, 1997)
R.J. White Prize for History (Downing College, 1997)
Charles de Gaulle Scholarship (Charles de Gaulle Trust, 1995)"
Lol..
On the other hand this guy got into the same chambers, (although he has the lowest credentials of any member):
"EDUCATION
LLB, King’s College, London
BCL, St. John’s College, Oxford
Bar Vocational Course, Inns of Court School of Law
OTHER ACADEMIC ACHIEVEMENTS
Kennedy Scholarship (Lincoln's Inn 2003)
Sunley Scholarship (Lincoln’s Inn 2000)
Hardwicke Scholarship (Lincoln’s Inn 1999)"
To get a place at some of the "less competetive" chambers, you really need a 2.1 and good advocacy skills. At the top ones, you need to do something special.