Some random thoughts.
David's eyes are strange. They are blue but in some light look very dark, as if perhaps he's wearing contacts. It makes him hard to read and distant/scary where he is less so in other scenes.
The date on something (PC/document/TV? sorry, forgotten what it was now) was October 2018. The top secret document Julia was reading on the tablet said something about serious sexual assault. I've gone back on iplayer and it says the name Charlotte Foxfield. There is a description of what the attacker did to ?her. Also before that "cocaine use is daily". Maybe info about the PM as Kitty, Knots suggested.
What is the red pin David and other protection officers have in their lapel?
I found it strange that the auditorium was silent after the bomb exploded. Wouldn't there be screaming or sounds of people in pain?
I noticed when David was scanning the room before speeches began, he looked at lots of females (including the policewoman. The only males he seemed to focus on were the press.) This might link in to some people upthread suggesting it could be the female cop.
They did make it look like he may have OK-ed a bomb in the case. If Julia has said it's best if she gets a new PPO, this event could be his last chance if he wants to assassinate her or be involved in an attempt. I still don't really think he is involved though, even if he's had thoughts about doing something. He's had multiple chances (driving her to Chequers, in hotel alone together) although I suppose just letting others do the deed there is less possible outcome and blame for him.