The government didn't force them to close, however; they just announced it by themselves. If they had physically been forced to close, then they likely wouldn't have announced a closure until Friday night.
I could personally see the parks opening up again towards the summer; the ideal breeding temperature for coronavirus is apparently 7-11°, which is exactly the sort of temperatures that Britain experiences on average in March/April, so it might spread a bit more slowly once summer comes, meaning that they feel safe to let us back outside again. In fact, if the park opens in summer and has a good Q3, dare I say that they might actually still match or beat 2015 in terms of attendance; every time I went to the park in 2015, I'd hazard a guess that staff probably outnumbered guests!