OPR of Gorbals, Glasgow has a Charles Mitchell of the 1st Battalion of Rifle Brigade marrying an Isabella Anderson on 29th January 1820.
The Battalion was based at Gosport but sailed to Scotland in September 1819 landing at Leith then marched to Glasgow where they were based for the next year. The Battalion left Glasgow in November 1820 for Port Patrick in Ireland. During the following seven years the Battalion was based in several places in Ireland before returning to Devonport.
If this is your ancester, he probably bought himself out of the Battalion to return to Leicester with his young family.