Yes, but not particularly harshly. The Electoral Commission's target is 95% of eligible voters enrolled, and it's currently at 93.5%. You are supposed to enrol at some time between your 18th birthday and the next election (which roll around every 3 years or so), so some of that non-enrolled percentage are folks who will do so shortly. A small amount of people are eligible to vote, but not capable, such as people with severe intellectual handicaps. In practice, most folks are enrolled.

The penalty isn't particularly harsh ($20 first time missed a vote, $50 afterwards). I have a 40-year-old friend that's not enrolled, and as far as I'm aware, she has never been fined for it.