It is true that as Christians it's our job to pray for those in authority. As Schaeffer might say, that's true truth, but it's not exhaustive truth - it's not the whole story.
For a start, who is "in authority" in our country? Is it the ruling party? What became apparent over the last few months is that hitherto, it has actually been the whole political system. The government, which doesn't just consist of the Conservatives, but also the civil service, exists (or existed) under the authority of parliament, the judiciary and the crown. When the governing party doesn't have a majority, that's not some mistaken fault in the system, that IS the system - it is how the system limits the power of a party that does not have a mandate from the electorate. Rather than pursuing policies that are divisive and don't have universal support, a party in that situation should be constrained to policies which command cross-party support. This system of government is what is in authority over us, and what we should be praying for - not just Johnson, not just the Conservatives, but for the whole structure that has been put in place by God for our good, of which this incarnation of the Conservative and Unionist Party is no more than a passing note.
One of my concerns about the Johnson regime is the way in which this structure was treated by him and his regime. Rees-Mogg lied to the queen - despised the crown - regarding the prorogation of parliament, the party sought to curtail parliamentary oversight, Johnson said the supreme court ruling was wrong. On multiple occasions, the Conservative party used executive authority to override the mechanisms of the state, specifically to prevent damage to itself (by suppressing reports which government mechanisms had produced). In other words, the Conservative party has sought to overturn every major component of our system of government, to guarantee their own authority. This was also included in their manifesto, and is what they are setting about now, in "reforming" the civil service, and with their plans to make high court judges political appointees. This is not democracy, this is not the authority of the system: it's the opposite, the usurping of that authority.
So what should we do? Who or what should we be praying for? I genuinely believe that in supporting the Conservative party as the ruling party, we are actually supporting a party which is seeking to overturn our government. The argument that we should not oppose those in authority, but pray for them, to my mind misunderstands the nature of authority and the rule of law in this country. It is not political parties that rule over us, it's the political and democratic system which has been established over centuries. The Conservatives are bent on scrapping this.
In a democracy, we have the right - in fact, as members of the electorate, the responsibility - to do something about this. We can oppose those who usurp power and corruption with our votes, with our legitimate protests, with commentary, in addition to praying for those in authority. We do not (yet) live in the regime of absolute power that Christians in the Roman empire did. In addition to volunteering for foodbanks (as Justin Welby suggests) it is legitimate to call out government policies which are resulting in poverty. In addition to supporting the homeless, we can point to what can be done to reduce homelessness. As Christians, we should not simply be socially and politically concerned, but we should not be less than socially and politically concerned.