I'm pretty sure the piece is a commentary on the recent phenomenon of people of a right-wing political orientation hanging up the England flag everywhere, to the consternation of local governments who have to spend money taking them down.
From a British perspective there's no ambiguity, flag shagging is a right-wing activity.
Every single left-wing march flies a lot of flags as well, only they are different flags.
Political movements in general don't seem to be particularly immune to flag shagging, only the colors vary a lot.
But I am pretty sure that Banksy means right-wing flag worship as well. He is a master of "provocative conformism" and wouldn't produce anything that would get him into a real risk of controversy. His art is very fine-tuned to the sensibilities of the English and American chattering class; same recipe for success as Paul Krugman or Malcolm Gladwell.
Is it? Most people I know who have flags proudly displayed are left wing and their flags are usually one of: the Palestinian flag, the ukrainian flag, the LGBT rainbow flag, or the trans flag.