Just in case, what line of succession
The Foreign Secretary, Dominic Raab is standing in for Boris. He seems a steady pair of hands, but lacking Boris' charisma and pizzazz.
I don't think there is a recent precedent of a Prime Minister dying in office. Sir Henry Campbell-Bannermann died 3 weeks after leaving office in 1905, but his successor was already in place. Before that you have to go back to 1865 when things were done very differently.
You should note that the British people do not vote directly for the Prime Minister. We vote for our member of Parliament, and the leader of the Party with most seats is invited by the Queen to form a government and becomes the PM.
So in the event of a PM dying in office, there will not be a General Election. There would be first of all a ballot of Conservative members of Parliament to establish two names that are acceptable to them. Then all members of the Conservative Party get to choose between the two candidates. This is what happened after Theresa May resigned.
The last thing the country needs at this time with Brexit yet to be finalized and Coronavirus at its height, is all the members of the cabinet competing in a beauty contest!! My guess is that the cabinet would rally around Raab until such time as the virus has passed its peak and choose a new leader after that. But still, you don't want the cabinet eyeing each other up and jockeying for position.
So please keep praying for Boris with all his faults.