I've got to disagree... time and again I'm being told (at Shefield Hallam) PRIMARY MATERIAL, PRIMARY MATERIAL and PRIMARY MATERIAL are the necessary and required sources. In all honestly - during semester 2 I have probaly only referred to a textbook in Contract and Consitutional a handful of times - I can't remember the last time I did so in Contract? I would agree - Comparative law, for example, is very much a different game altogether - textbooks are my main port of call but again, we're told that in our answers it is reference to the primary material (direct reference to the consitutions and the formative cases) that they are looking for in the exam.
Practically all of my revision and semester 2 notes have come from Statutes (UCTA 1977, SGA 1979 and SGSA 1982) and cases... textbooks and journal articles have been referred to a handful of times.
Constitutional - CASES, CASES and the PUBLIC ORDER ACT 1986... Several journal articles for Jackson, Factortamé, Thoburn - but no text books AT ALL. In fact, I picked the text book up last night, leafed through the section on Parliamentary Sovereignty and tossed it aside. I'm dying for a question to come up on Parliamentary Sovereignty because I feel my thoughts, reflections and opinion on the concept go far deeper and further than any text book or journal article. I've pulled bits form all over to develop this line of thinking but the majority of it has come from reading the judgements in numerous cases but especially Thoburn and Jackson, considering it all in relation to Dicey's view - the time at which Dicey established his opinion and then considering where the ECA 1972 (esp. Factortamé) and the ECHR/HRA1998 have left us; even bringing in the "Poll Tax" issue. Very little of that has come form a text book... its come from reading the primary material and tossing it all around in my own head.
I don't think I've once been overly encouraged to find the answer in a text book. Even in Comparative, when I have emailed the lecturer with a question, 9 times out of 10 he sends me back a long answer coupled with a link to a relevant case/s on the Findlaw site.
So I do disagree with your implication that non-oxbridge universities do not emphasise primary materials.
**I lie - I do remember the last time I called on text books in Contract - DOZENS OF THEM!!! SGSA 82 and the SG 1979 and how they operate when there is a sale of goods and a contract for work - few of them were of much help... CWTF's book did help immensely! In fact, I still have it and it is one textbook that I will be referring to in my revision on SGA/SGSA/SSGCR!