The achilles heel...
... of the drone is the comms uplink required. It's a neat tradeoff though: Kick out the pilot and you suddenly care much less about bringing home the airframe in one piece, or at all. So it doesn't need to be fast or agile, it just needs to be _there_ and you need enough warning to fire off whatever counter for whatever's incoming the thing is carrying.
But that doesn't work so well against, say, anyone with a jammer (as already mentioned). Or just plain shoot the sats that make it happen out of the night sky. Then what?
Of course, shooting satellites out of the sky is not something every puny little country would manage. But there probably are a couple that would if pressed.
It is certainly interesting that where fighter design has solidly derailed in multi-decade morasses of computer modeling and budget overruns, these far simpler drones could be shaken out of a hat inside a couple years. Probably to do with unwillingness to have their own people killed. As in, Rutan's famous remark. "The West[tm]" has stagnated in that respect. A jolly old all out war would fix that right quick, but modern weapons are so destructive these days that nobody dares think of that. So "we" make do with picking on someone not a tenth of "our" "size" instead.
Why? Well, the military-industrial complex needs to be kept busy, of course. Also why "homeland security" is the ultimate selling point for everything from software to combat boots, and a lot of other things besides. Like dowsing rods.
The debate drones vs. fighers is thus mostly one of vested interests vs. vested interests. In the end, a complete changeover to drones is probably unwise in the same sense that the "drop the cannon, we've got missiles now" mentioned above been tried at least thrice now, and been proven to be a Very Bad Idea Indeed[tm] at least as often. It's a useful addition as long as the infrastructure required to run it holds, but no more than that. Sometimes you just need people in planes much like you still need people on the ground, and you'll still need at least some of them even if you managed to build entire robot armies. War is much too much a human trait to still make sense with both sides or even just one side entirely dehumanised.
That infrastructure, though, is decidedly global, and bloody expensive to set up and maintain. It does far more than enable drones, but drones very much depend on both GPS and military comms sats, to the point of being entirely useless without. Should either fail, pretty much nobody on the planet can replace it overnight. There just isn't enough capacity to send it up even if there was enough reserve kit standing by.
The enabler for these things is the pinnacle of high tech and that is something that stands the highest chance to fail, vanish, or otherwise become unavailable, as soon as you get in a serious war. Now the Americans[tm], though having meddled in small fracas (for them) constantly, haven't had to fight tooth and nail for their very existence in a long, long time. And it shows. Blighty isn't much different, though on a smaller scale.
As a sidenote: Comes push to shove, we'll see how much they'll care about their "bestest friends". This might be less than expected, though that has next to no bearing on current Blighty acquisition as the USoA has to aquiesce bloody everything already anyway. Then overpriced local assembly is just that much propping up of ailing economies that should've reinvented themselves long ago. To fix that, you have to have better options than "buy (export-rated) american" or "buy european (full of export-rated american tech anyway) for more and get less".
If there's any point it's that military spending for any sake but its core business, be it the economy, politics, or even the seniority game, is equally valid, which is to say, not very. The core business of the military must be maintaining capability of and excercising force projection (no point pretending it's just for "self defence" but that's covered here too), and procurement must thus be about getting the right kit for the job. Now you can debate just what exactly the job is, but it _isn't_ lining pockets or keeping people "in jobs". If you can't manage that simple bit of focusing your priorities you might as well just give the squaddies the money and send them on permanent vacation. It's so much cheaper, and good for quaint pub maintenance and such. The tax benefits might be even better as alcohol comes with extra duties.