There has often been a self-assurance verging on smugness by some British commentators regarding how safe the UK is from terrorist attacks compared with our fellow nations on the continent due to our nation not being in the Schengen zone. It is true that there is a much greater chance to smuggle weapons and explosives into the Schengen zone compared to the UK. You only need to cross one border in Eastern Europe to be able travel anywhere from Seville to Stockholm to Slovenia in theory without being checked any further but entry to the UK by Car or foot is only achieved through a few highly secure port entry points such as Dover, Portsmouth and Plymouth.
But whilst it's true that the UK is safer from the threat of smuggled weapons and explosives than Europe, it is not as safe as it was. There has been an increase in the cases of British citizens paying for guns and explosives to be sent to them from abroad to the UK via the dark web. Two cases of note are the would-be spree killer Liam Lyburd who bought a Glock Pistol and of Michael Shingler, who bought a quantity of triacetone triperoxide (TATP) the explosives used in the 7/7, Paris and Brussels attacks to attempt suicide with.
In both cases it is safe to say neither would have had access to the weapons without the dark web. One of the defining characteristics of British Al Qaeda/ISIS plots in the period after 9/11 is how of the hundreds of would-be terrorists who were convicted, how few of them had any realistic chance of harming anyone with anything other than melee weapons which as the Woolwich attack of May 2013 has shown can be dangerous but nowhere near as dangerous as firearms and explosives. A great deal of these plots were disrupted by the authorities due to the would-be terrorist's communications being intercepted due to shoddy tradecraft. The advent of almost entirely encrypted communications make it much more unlikely that some of those plots in the 2000s would be discovered today.
This is not to say, that the dark web or encrypted communications are bad per se. They are a big thorn in the side of coercive regimes worldwide, and there is a persuasive libertarian argument for their use if used for benevolent purposes. In any case you can no more ban these technologies as you could the surface internet or the printing press. It's important to note though, it could conceivably lead to a sizeable increase in the proliferation of weaponry and terrorist expertise to those with motives but previously no means.
It would be a democratization of terror so to speak. This isn't to say there wouldn't be ways of fighting back as law enforcement agencies in this and other countries have been launching sting operations to trace and arrest the buyers and sellers in this market and in an economy where one can't report fraud to trading standards, being scammed is a real possibility. Nevertheless, there is a viable market in ordering items more commonly seen on the battlefield via the dark web to be ordered hidden in the post and ISIS have been making use of the dark web for propaganda and communication purposes since late last year.
We can no longer reassure ourselves that we are 'fortress Britain' impervious to the chaos we saw engulf France and Belgium in the last eighteen months. It's no good to reassure ourselves smugly that what is over the channel, can't cross over. Yes, you can't smuggle an AK-47 from The Schengen outer border to London with no checks in theory like was the case in Brussels and Paris, but if you were street-smart (or 'web-smart' to be more exact) enough to discern successfully which were genuine vendors of these goods, you could order the parts for an AK-47 to be smuggled to London from Eastern Europe disguised in seemingly innocuous packages. If Lyburd hadn't been arrogant enough to post about his plans on social media beforehand, he quite possibly could have carried out his plans of mass-murder and mayhem with weaponry bought online without detection.
For too long, we and others have looked at what has happened in Belgium and France as proof of incompetence, condescendingly stating it could never happen here. Whilst mistakes have been made by the security services in those countries. A lot of what occurred, would have been much easier to detect 10-15 years ago and a lot of the UK's near misses from that time period would have been harder to detect before that date as well.