But sometimes it seems less like they are preparing for a threat and more like they are hoping for one.
That is so true. And of course they have to justify their own existence. peacetime is no fun for military organisations.Not that we've had any in recent decades.

(bearing in mind that by 2035 3D metal printing in the home will probably be commonplace) .
Hmm, doubtful - but if someone comes up with an idiot proof and non-dangerous way of doing it. yep it's possible.

Metal printing in a small workshop - should be far more common. So yep firearms could be easily and anonymously manufactured inside a countries border controls. That's a fairly serious potential issue for the uk.

Fortunately terrorists tend towards the unimaginative. Anyone who's prepared to kill themselves definitely has their imagination centres in their brains blocked off, or they just wouldn't do it.
As a person with a very active imagination - be glad I'm not a terrorist. Just so much stuff you could get away with that doesn't involve suicide or explosives. Or hell just making the right kind of explosives, ie: ones that won't blow up during manufacture.

So I'm not sure that 3d printing is going to have any real impact on terrorism in the uk.
Big-data - well it does massively increase blackmail potential for people who put their entire lives online - you know, idiots.

But if you want to worry about big data - then worry about the companies who collect and use it: google, supermarkets, banks etc.
Again not terrorism as such, just regular every day scumbags.

And personally the one I do genuinely worry about is google.