Is that even legal in the UK? As far as I knew, UK has a statutory 20 days off for all salaried workers (28 with holidays). Is it 20 days off plus unlimited? Which isn't quite the same.
So I don't think it would be an issue in the UK either, but the company needs to do the accounting right.
Many companies offer better than that as an additional benefit. For example, I currently get 24 days + 9 public holidays (Scotland) for a total of 33 days. I think I get an additional day after I've been with the company 4 years.
It's a small company, so it's an honour system. I've taken far more than my contractual minimum every year I've worked here, so having to get backpay or carry over hasn't come up just yet.