It's standard. Actually pretty good here. It's meant to grow too, in most cases. Does UK PTO get bigger the more you work?
I have 29.5 days in total, with some of those being compulsory because the office is closed. Some are confusingly called 'compulsory discretionary' holidays due to the hilarious management I have.
My employer adds an extra half a day each year you stay with the company, plus another half a day if you meet sickness attendance targets.
But it also depends on which union you're a part of etc.
In my experience, different companies handle it in different ways - one company I was with increased it by five days, but on your fifth anniversary with the company. I've also heard of companies giving one-off extra holidays for long service or other achievements.
If you're employed directly by a large company, your holiday situation is usually pretty good in the UK. If you're employed through an agency or you're a temporary worker, then it's usually pretty dire (compared to the UK norm - probably still quite good compared to the US).