I can't see any reason to not be at circuit height by the time you turn downwind... or at least by the time you pass abeam the upwind end of the runway. If necessary, extend the upwind leg. I was taught to turn crosswind at 500' agl, which pretty much guarantees you'll be at circuit height in time.
As you say, not being at circuit height by this point introduces a risk of collision with joining traffic. And that applies on both sides of the pond, with a 45-degree join as well as with a standard overhead join.
Traffic considerations apart, turning at very low level doesn't seem safe. Build up speed, get a bit of distance between you and the ground, get the flaps up - then think about turning crosswind.
FFF
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