I was thinking of people who are employed part time because they can't get a full time job - or at least not one that suits their needs.
William Joseph1 wrote:People are welcome to do more than one job if they feel under-employed.
Sadly, it's not that easy. Variable hours and variable shift patterns can make it impossible to fit a second job around a first. I recently met someone who was assistant manager at a branch of large fast food chain. Because she couldn't commit to 24/7 availability, she had a zero-hours contract, which gave her about 20-30 hours a week. I'm sure she would have loved to predictable and regular hours.
I'll admit that my assertions about underemployment were based on anecdote. The ONS tables link to a report - itself nearly 10 years old - which discusses the difficulties of defining underemployment. If read it correctly, they point to international definitions of under-employment, which don't capture well the type of scenario I mentioned.