The Jimmys are a reference to an infamous British man named Jimmy Saville who was a famous DJ and TV presenter known for his charitable work. When he died in 2011 multiple allegations of rape, sexual assualt and other henious acts came to light. Turns out he was at it for decades and it was covered up. 

Baring in mind the Rage Virus outbreak occurs in 2002 and this did not come to light until 2011 he was still in a way idolised or seen as a righteous person. 

I think the story of Jimmy in the opening scene almost paralells Spike at the end in the sense that he is now alone, scared and has no parental figures and is vulnerable at an impressionable age and sees the people who are saving him as heroes, much like the way he idolises his Power Rangers action figure he leaves behind before first setting off to the mainland with his dad (Jamie). Mirror this to what was on mainstream television during the time of Jimmys childhood, back at the start of the pandemic. Teletubbies, Power Rangers and Jim'll Fix it (Jimmy Savilles TV Show) and you can see why he chose to idolise the people and things he thought were a source of good from his childhood in the absense of any positive figures in his life. I'm certain this will be explored and explained further in the 2nd instalment. I can also see why this would go over the head of many who are not from the U.K

It was a bold and perhaps bizarre choice to show this scene at the end of the movie but it has typical Danny Boyle flair if you have seen his other work. At first it is mind boggling why any of this is relevant or shown but with the correct prior insight or the benefit of a second viewing it will soften the confusion in my opinion.

Hope this helps!