1962 OS map
here shows that the road layout has changed, and looks as though there was plenty of room to do a full circle (almost around the word 'OLD' in Old Bexley Lane) without the need to reverse, setting down and picking up at the shelter on the north side of the road. There is a (now out of print) enthusiast produced book of diagrams of LT country bus terminus arrangements (as far as I know there isn't a central area equivalent) and this confirms that short working 401's did that.
I'm not aware that the arrangement changed after the 124A went OPO, but I can't say that it didn't.
Other u-turns I can think of on the London network -
trolleybuses at Stamford Hill Broadway - not sure if this continued in to bus days or if so for how long;
trolleybuses and later trolleybus replacement routes from the north at Liverpool Street, Bishopsgate - not sure when this ended, but think 149's continued to do it in to the 1980s;
also at Manor House, Seven Sisters Road, from the east (think this lasted quite late - picture, not mine, of an Eastern National VR doing it in 1987
here.)
There is also
this arrangement on Waterloo Road for buses to do a u-turn with traffic signal assistance - it's been there at least 10 - 15 years, but not sure when it started. May have been when the Cornwall Road bus stand turned in to the Red Arrow garage.