If you're new to the airfield it makes a lot of sense to fly wide around the circuit (possibly even outside the ATZ) and join in the prescribed manner. Which in this case seems to be a direct downwind join.
The instructor may well have a lot more local knowledge and know from experience which shortcuts are acceptable and which are not.
Furthermore, was this an A/G, AFIS or ATC airfield?
Personally I am always wary of non-standard things at an A/G field because radio usage seems to be relatively poor - not to mention the no radio aircraft around. But with ATC fields you can generally ask for anything as long as it doesn't cause a conflict or noise abatement issue. Where I trained (KISM, full ATC) they had both left and right hand circuits in operation simultaneously for one single runway as a matter of course. Which circuit you got was determined by which direction you came from and nothing else. Worked a treat. If you were to join final behind an aircraft on the opposite circuit and you didn't see it, you just had to say so and ATC would call your base turn, and so forth.
It was a typical G/A field with call sign "Radio". Having said this, I'm used to flying to similar airstrips here in Germany - Koblenz, Melle both spring to mind - which have both left and right circuits in use and am ok to being advised which to join, however I have never gone to an airfield before, be told "left hand circuit in use" and be expected to join right downwind, hence my question - I wondered whether this was a UK thing, that you can receive "airfield information" and then ignore it......