No one knows for sure how long immunity lasts after being infected N_ though some estimates put it around 6 months. Also, no one knows as yet to what extent immunity from having been infected with one variant transfers to other variants. Same goes for the vaccines mind but there is evidence that they are protective against the current variants. It makes sense for him to be vaccinated.
Aside from that it seems a bit daft that the Prime Minister is in a queue for vaccination. Surely him and probably all MP’s ought to be prioritised so thay can get on with their business. I suspect he has already been but is trying to encourage vaccine take up with the public.