There’s an unwritten law governing Boris Johnson in Westminster: every-thing he says or does is a gaffe, or can be portrayed as one. Yet actually Johnson has an uncanny knack for conjuring similes which sum up the political situation precisely. So it was for his much-ridiculed remark, in response to a question about the Irish border, that there are no border posts between London boroughs even though they have different business rates and policies on various other things.