Christmas Eve Jubilee (Hungerford) bridge.
Established since about 2010, originally a seven piece band, now usually four or five. On Xmas Eve it was just four (three at the time I arrived as one of the boys took a break see above picture at left).
They are local lads from SW1 (Lambeth/Southwark) who are regular buskers on the Jubilee Bridges or about the South Bank.
The band has made appearances at international festivals in Sweden and the USA. A couple of pics from their Facebook page showing the original seven line up:
Secret Sailing Club: Facebook / Twitter