In Zimbabwe, I was told a 375 H&H Mag was the lowest calibre permissable for DG.
I had a one shot kill on a pretty big bodied Lion using a 375 H&H Mag, with a Kynoch soft.The shot was at 120 yards and was over a Zebra bait.
So from very limited personal experience, I would go for the 375 H&H Mag.
Having seen the Lion charge on the video 'In the jaws of Simba', I would personally go for a heavy double(450 upwards) for a similar situation.
M.