You can get freelance as an artworker but you do have somewhat of limited audience as you'd need to target design/ marketing agencies rather than clients directly.

Having said that if you're happy to travel about working in house for agencies, and are a genuinely competent artworker their is no reason why you couldn't make it work. The last thing most agencies want from a freelancer is someone who thinks they are a rockstar designer. A creative artworker is prob the most desirable, someone who can quickly and accurately set up artwork but also take an initial design direction and flesh it out a bit.

You may find it difficult to keep your current role and get out a lot of out of hours work however. Deadlines and practicalities often mean agencies would prefer artworkers to be onsite and during normal office hours but this will vary on the jobs they need doing

As for finding these design/ marketing agencies just google it for your local area/ the area you would be willing to travel to, they should not be difficult to find. Also worth searching for and signing up to recruitment agencies that deal with freelance work, quite a few around.