Yes. I've supported myself as an illustrator full-time while living in London, though now I've moved into a more general creative consultant role because Illustration has no real career path, but I'd still say I'm a pro; my list of past clients includes Nintendo, Screwfix, the BBC, a bunch of events, government authorities and large charities and organisations etc. and my job still involves making art in a professional capacity from time to time (most recently helping on some art for a show bible for Cartoon Network Execs and other Bigwigs).
I think the term "semi-pro" is underused. I'd say anyone who has done a few bits that earn them a decent bit of money on the side could reasonably call themselves "semi-pro", and that's still pretty impressive. I'd say a "pro" is someone who has managed to support themselves full time for a couple of months with just art. Once you're a pro, with art it's kind of different from say football because if you're not doing it any more, you're still kind of a pro 'cause you can still bust out pro quality art any time unless you really really let your skills go rusty to a terrible degree.
There's no shame in not being a full-time pro and it's definitely not for everyone. As jobs go, it doesn't pay well and often involves drawing a lot of really boring stuff for other people to the point that you have no energy to make anything of your own any more. If you want to draw just stuff you care about at your own pace, do a different job to pay rent and then enjoy making whatever you like. There's such a sense on the scene that if you haven't "broken in" you're somehow a failure as an artist, but the number of people I know who went pro and were just miserable and stressed out and so switched into working in games or design or consulting is staggering.
If you do want to be a pro though, the most important thing is being seen by people, so you need a nice website with an easy URL, strong social media presence, to go out to events and give out lots of business cards, and to be a good part of your art community, supporting other artists (my big break was a job for Nintendo. I got it when a more senior artist was too busy for the job and recommended me because she felt like I was a nice and reliable person based on my behaviour and output on the UK manga scene).
You also need a flexible art style with decently broad appeal and a professional level of polish. You may not always get to draw in your "favourite" style, and hey, if you're an elitist, you may be thinking "hah! Yeah, silly kiddies who draw manga, you can't just draw manga all the time!" and it's true, but hey it actually works both ways. I've seen artists who insist on drawing really sketchy artsy bleak stuff getting stuck due to lack of demand for that (most of them become jaded art teachers
), you need to be able to draw well in at least one style that has broad appeal in the context of say newspaper or magazine illos, adverts/websites or comics (and I mean comics that have followings of 10k plus people, like manga, marvel/DC superheroes, newspaper funnies, not like "oh I draw like the artist who did this one graphic novel that critics praised but nobody bought").
Being pro isn't about being the best artist in the world so much as being consistently, reliably good, easy to work with and supplying work in styles that there's demand for.