Though I will say there's possibly a good reason why so many aren't registered.
Looking at my own circumstances, I still qualify (just) as an overseas voter, but I haven't registered in years. I did used to, but as time moved on I became more and more detached from UK life and UK politics. As a dual citizen, I exercise my vote here religiously. It's where I live and work and Colorado and U.S. politics affects my day to day life. But the idea of voting in a UK election - when at best I'll spend a handful of days a year. It just doesn't feel right.