"Because of this there is not always a sitting Deputy Prime Minister. This means that it is up to the Prime Minister to choose whether or not he/she wants to have a deputy. The position was created in 1942 and Clement Attlee was the first Deputy Prime Minister. There has been no Deputy Prime Minister since 2015." - Simple wikipedia
Boris Johnson has just been admitted to hospital due to COVID-19. It feels logical that it should be someone high up within the Conservative party, but I don't know who it would be / what position and a quick search for this didn't yield any results.