Some jottings from the military records that I've been able to find for the John Shields who served in 81st Foot and various veteran battalions:
John Shields appears on the muster roll of 1st Battalion 81st Regiment for 1Q 1814 (then in the Peninsula). Regimental number 443. Had served more than 7 years.
Peninsula medal roll: present at battle of Maida (1806). This medal was only awarded to claimants still surviving in 1847. This would have been 1st Battalion, 81st Regiment.
7th Veteran Battalion: discharge records shows that he had served 13 years and 3 days with 81st; an out pensioner from 25 May 1817 to 3 November 1819. Attested for VB at Keith on 4 November 1819 and served with 6th & 7th. Discharged from 7th VB on 17 April 1821 as a consequence of its disbandment. Born Hanbridge, Cartmel, Lancs, circa 1778.
3rd Veteran Battalion: Discharge record shows “7th Service” from 18 April 1821 to 24 May 1824; 3rd VB from 25 May 1824 to 14 February 1825 (attested at Sheerness). Discharged in consequence of “chronic catarrh, increased since last brought up and unfitting him for guard duty”
81st regiment locations:
Assuming he was in 1st Battalion throughout his time with 81st regiment, he would most likely have served in:
Italy 1804
Sicily 1806-1812
Spain 1812-1814
Canada (Quebec) and USA expedition 1814-1815
France 1815-1817.
7th Royal Veteran Battalion: various online histories assert that this unit was disbanded in 1814 or earlier but contemporary newspapers locate it in Guernsey and Jersey in 1820/21.
3rd Royal Veteran Battalion: was in Jersey 1824/5.
Pension records:
1845 transferred from 1 Divi North London to 2 Western London
1849 South London
1855-1864 residence South London - there is an annotation in 1862 which I can't quite decipher
https://search.findmypast.co.uk/record?id=GBM%2FWO23%2F55%2F00161&parentid=GBM%2FWO23%2F55%2F00161%2F1217416Although pension records place him in London until 1862, I haven't been able to find any census entries for him. Did he perhaps die earlier and his widow failed to notify Chelsea?