Michael George's obituary notes that he was a Waterloo veteran.
The Waterloo Roll (comprising over 45000 names) contains no Doubell names - the closest appears to a name listed as George Diabble in transcribed lists. However a copy of the original hand written list is too poor to be certain of the name - and it could possibly be Doubell - or a variant of the spelling. If this is Michael George, then he was a private in Captain Eele's company of the 3rd Battalion of the 95th Regiment of Foot (the rifle Brigade).
This is not impossible. The regiment was based at Hythe in Kent and was famous from early exploits in the Peninsula war and so popular that in 1809 it was over-subscribed by 1000 recruits so formed the third battalion, which joined the others in the wars against Napoleon. Being local in Kent this would have been an attractive unit for Michael George Doubell to have joined - probably under-aged. The activities of Eele's company at Waterloo are described here https://simonjoneshistorian.com/2015/06/17/a-rifleman-at-waterloo/.
After Waterloo, the 3rd Battalion entered Paris, were sent to Ireland for a time, and were then disbanded in 1818. Many 3rd Battalion men were transferred to the 2nd Battalion, which was then sent out to Malta - any ship would most likely have stopped at Gibraltar on the way, which would fit nicely with the story of Michael George having 'jumped ship' there to head off to South Africa.
Any thoughts anyone?