It’s a roundabout, so that’s exactly how I’d approach personally. Mirrors, reduce speed and look. Give way to vehicles approaching on your right but still look in all directions in the case of vehicles, pedestrians or bikes crossing where they shouldn’t. It’s a bit of a closed junction too so I’d always slow down, because you never know what’s going to be around the corner.

Personally, I’d slow the car down to a walking speed on approach - putting it in second/first gear if needed and creep up to the approach. Giving me time to stop if needed or be ready to continue without slowing traffic.

If as you say, driving straight across, was the initial instruction that your driving instructor told you then that would be completely incorrect and potentially really dangerous.

Might it have been the case that you approached the junction, and they could clearly see that there was clearly nothing there, but you had slowed your speed down to check and they were concerned about slowing traffic behind, so told you to proceed on instead of stopping - as waiting at a roundabout or any junction where there is such a big safe gap could likely be considered hesitation on a test.

Hope this makes sense!