The reddit thread about that article has almost 10,000 posts on it so far. They've been discussing 2 recent videos where he spoke of his condition and sounded short on breath.
They also mention he is overweight, he's old, and he had childhood pneumonia (which they imply make him higher than average risk)
Then this was posted suggesting he is on the part of the timeline where Sepsis and/or ARDS would kick in.
note, I'm not saying they are 100% correct, just trying to share a TL;DR style recap of the high points I saw in that thread. If the Prime Minister pulls through without hitting sepsis, ARDS, or needing to be ventilated you can mock me lightly and I'll take my shame. If he gets worse I won't shout I told you so, I'm not claiming any great insight.