Up until 2005, there were no restrictions on dealing with plasterboard or gypsum waste from construction sites. In that year, the government (at the behest of an EU directive) began to treat plasterboard differently. Although it's not a hazardous material in its own right, it can react with biodegradable materials to create hydrogen sulphide (think rotten eggs) and it has been deemed to be best if plasterboard isn't just added to landfill where it might encounter just about anything.
But the regulations have never had much teeth and, unlike asbestos, waste companies have tended to turn a blind eye to the disposal of plasterboard and gypsum plasters generally. This is because builders themselves either don't know about the requirements for plasterboard disposal or, more likely, don't care. In fact, plaster products are recyclable and a good waste disposal business will do just that, so that very little plasterboard need ever end up in landfill. But a skip user has no idea what will happen to the skip after it leaves site. Plasterboard off cuts just get placed in the skip, or taken down to the tip.
In fact, my local skip company does offer a plasterboard-only skip collection service. The special skips cost around 15% more than regular skips, and in addition charge of £75 for each extra tonne above the first tonne placed in the skip. On the other hand, they readily accept plasterboard offcuts in ordinary skips as long as the plasterboard makes up no more than 20% of the waste. Who exactly is going to measure that?
Plasterboard buying is notoriously difficult to estimate correctly because it produces so many offcuts. It is usually better to slightly underestimate and then buy the additional requirements in small batches until the work is complete. What you want to avoid is having to dispose of 20 or 30 sheets of the stuff.