I have literally answered that a few posts ago. Strangely enough, in a country that doesn't have many guns and most people never even consider guns, we don't have accurate figures for "how many illegal firearms have you seen in the past year" statistics.
Tiny is quite obvious. We don't have guns. Unless you are rural or in a really bad part of cities you probably never see guns legal or otherwise. I lived in the country and we had a range close to us for clay pigeon shooting. Outside of that I have never seen a handgun in person apart from on armed police. I have lived in London for years and never saw a gun on anyone but the police.
If you are struggling to imagine it, imagine that no one you knew had any personal firearms unless they hunted (which isn't common in the UK) and that your police didn't carry guns and your criminals didn't have guns. Thats pretty much what its like in the UK.
Just for info, on the last count there was approximately 600k section 2 (shotgun certificate) holders and 150k section 1 (Firearms certificate) holders in the U.K. - some people have both so you can’t add them together, but probably about 650-700k people with legally held gun in the U.K. Between them they hold 1.3 million section 2 shotguns and 250k section 1 firearms which includes everything from AR-15s, cannons, mortars, flare guns to tanks (yes you can have a firing tank in the U.K. so long as you have a reason and it’s not self loading). So approximately 1.55 million guns in a pop of 65 million in the U.K. - 1 gun for every 42 people.