Consideration would need to be given to checks on people boarding tube trains

Spot the person who has never ridden a rush hour tube. People don't board the tube, it's more of a pouring action.

Hyperbole asside, I can't see any practical way to vet 3 million people boarding tube trains each day. The best you could manage is some sort of token random search, a bit like ticket inspectors campling out at random stations. Anything like that would be far more for visibillity than actual security.
The only thing I can think of that might help would be a practical explosives sniffer, but even if one was devised which worked properly in the real world it would be a huge job to implement and maybe just push the prblem elsewhere. Cf "technical fix" passim.
We should point out to him that if we have security checks before boarding the Tube, terrorists will simply blow up ticket halls instead.
Or a shopping street, gig, nightclub, church, exhibition, school, motorway services, ferry, crowd leaving a sporting event or any other congregation of people.

There is nothing we can do to prevent the risk of this on the ground. At best we might get lucky with some sort of search procedure. The only systemic chance we have of preventing these things is to hear about it in advance, or to prevent people and equipment from getting into the country in the first place. Except of course a lot of them appear to be British anyway, and if you know what you are doing buliding your own bomb isn't all that hard.
Gods but he's a prat
"ID cards won't work against this either, but we still want them anyway, because, erm, they might prevent something, but, erm, it's a secret and I'm not going to tell you what".
I've always maintained that ID cards would never make a shred of difference to the way terrorist groups operate. I don't pretend to understand even the reasoning behind the scheme or how the cards are expected to work against crime and terrorism. I've always thought the idea was entirely ludicrous.
It seems to rely on the idea that the perps are going under aliases and strong ID will stop this. The trend now, though, is for them to operate under their own names and rely on being able to find people who haven't been in trouble before. The people responsible for the WTC attacks and the Madrid bombings all had perfectly good legal ID because they weren't pretending to be anyone other than they were. I'd be interested in seeing whether the same is also true of the Bali bombing. I suspect it is.
Maybe they will be flexible enough to place over a chest wound and stop a lung collapsing.
This notion ignores the game-ability of ID schemes, and because the average political hack has no understanding of higher reasoning like game theory, explaining it never seems to work. They can't seem to grok the idea that I could theoretically recruit a few dozen terrorists for a mission, send them through the new ID checks a few times to see who always gets stopped, who never gets stopped, and who gets stopped initially but can then penetrate security when their faces become familiar, and design an operation based on the threat model the defenders just revealed to me. You pick your attacks or you pick your targets, never both.

Honestly, I'd pick a softer target if I was a practical terrorist. McVeigh was successful in part because nobody could imagine a white former soldier blowing up a building with a daycare in it in the middle of freaking Oklahoma. It's not like the world failed to notice that bomb just because it happened in the middle of a shit of a city. Knowing that Tim McVeigh was Tim McVeigh, absent knowledge of what was going on inside his head, was a useless datum.

God, I hate security amateurs.

God, I hate having these conversations after an "incident." Everyone things I'm much farther gone than I actually am when I talk like this. :\
So, what you're essentially saying is - that you have absolutely no idea how they work, but that they couldn't possibly help?

Sounds like my mum and mobile phones!
Nobody has ever said ID Cards would stop terrorist attacks though, just that it would certainly make things more difficult for that sort of thing to happen.