What is the little icon in the top left The icon disappears when I'm using the phone.
Giffgaff (stylised "giffgaff") is a mobile telephone network running as a Mobile Virtual Network Operator using O2's British network. Owned by O2's owners Telefónica, Giffgaff was launched on 25 November 2009.

It is the Branding logo from your Mobile Telephone Network

From Wikipedia