This relief is "The Death of Nelson" by J. E. Carew, on the front (south) face of the base of Nelson's Column. It depicts a scene at the Battle of Trafalgar, with the dying Nelson at the centre. The standing man at the far left is George Ryan, a Caribbean member of Nelson's crew.
Brought to our attention by Londonist.
Site: Nelson's column (2 memorials)
WC2, Trafalgar Square
Some of the earliest ever photographs are of this being erected. At the four corners of the square are plinths holding octagonal lamps which are said to be the oil lamps from Nelson's flagship, Victory. Note: At the south-east corner of the square there is an extremely small police lookout post. In 1826 there was a lamp then in 1926 a phone line and light were installed so the police could call for assistance. Sadly it is now a store room. More at Londonist.
This is not the only "Nelson's column". They were erected all over the world and Londonist has done the research.