Genghis,
Rules Of The Air, Section 4, rule 17, paragraph 5:
(5) Flight in the vicinity of an aerodrome
Without prejudice to the provisions of rule 39, a flying machine, glider or airship while flying in the vicinity of what the commander of the aircraft knows or ought reasonably to know to be an aerodrome or moving on an aerodrome, shall unless, in the case of an aerodrome having an air traffic control unit that unit otherwise authorises:
...
(b) make all turns to the left unless ground signals otherwise indicate.
For a left hand circuit, the 45° join (how do you get the ° sign without copying+pasting from your post?!) requires a right turn from the 45° leg onto the downwind leg, which goes against the above rule.