The Northern Ireland link was easier to find:

> You must take at least four weeks’ holiday a year, so only holiday on top of this can be carried over and then only if your employer gives you permission or if this is permitted by your contract of employment.

I once had a colleague who was given a letter along the lines of "you still haven't used enough holiday; unless you advise us of your preferred days, you will be on holiday from date X to Y".

https://www.nidirect.gov.uk/articles/taking-your-holidays

It's a requirement to take the legal minimum (20 days), but most companies offer 25 or so so in practice you can usually carry over 5.

I believe it's in order to nullify the ability for a company to apply alternative pressures to stop people taking their entitled holiday.