Or unless you happen to be Alex Henshaw in a spitfire!
I wouldn't expect that even/especially someone of his experience would do a barrel roll this low. The margin for error is just too small.
What you will often see is an aerobatic figure done at quite a high altitude but the exit dive is allowed to run out quite low for effect. e.g a loop where the normal pull out would be done at 500 ft but the pilot lets it descend much lower during the last 1/8th. This 'fools' most of the audience but gives a big safety margin for the pilot. I suspect this might be why you see "barrel rolls at 50 ft" in articles.
ZA