It is a hard one.
The Edward from Okney was born on 13 Aug 1829 and shows at home with parents in both the 1841 and 1851 censuses. I cannot see him easily after 1851.
Spence is not a common name for the Dumfries-shire area, and nothing really fits there for an Edward born there anywhere between 1830-37 does it
I suppose the main thing I am seeing now is that 1851 census entry for a young Private from Dumfries at the Chatham Barracks. It seems to be a good fit...however, making that link back to Scotland is hard as nothing really shows suitable in 1841.
The reference to Dumfries must come from his attestment papers you would think though. Why would the army make enquiries there if this was not showing as his home town on his papers. If nothing came from their enquiries, guess that means the Victoria Cross was never handed over to his family members? (Added: See that it is held and displayed at the Black Watch Museum in Perth).
Ian, have you managed to see his attestment papers from TNA?
You mention you are in Dumfries. Nothing there in the archives, not available online, that may help you further at this stage?
Monica