Not sure it's in any sense particular to the Britosh system. The US, in particular, seems riven by such tribalism. It may be that they were always like this, and certainly, I think the past had a lot of elements of it. That said, the existence of social media seems to have exacerbated it. The reactions I saw on social media to the arrest of Nicola Sturgeon yesterday seemed affected by it, for example.
I think though that Johnson more so than any other leader that I can think of in the UK is merely there out of ambition.
In the past, the US system was not tribal. You frequently got bipartisan cooperation. However, that has changed and it’s given them a lot of trouble. Their political system cannot cope with tribalism. The people who wrote their constitution assumed, wrongly, that the politicians would act reasonably and in the best interests of the USA.
Our political system has always been tribal and it has evolved to deal with that. We have safeguards in important places such as an independent electoral commission and a neutral Supreme Court. However, much of our system is based on conventions rather than explicit laws and so it can only deal with tribal politics as long as all the tribes’ members agree to play by the rules.
