Having had to decide upon the fitness of hundreds of football pitches in my time, I can tell you that (in the current weather conditions) if they've not been rolled after they were last played on there will be ruts which could easily result in a broken ankle. If they have been rolled they'll be too glassy to run on. The only times I can remember declaring pictures fit to play on when the temperature was around -5, with a wind chill down to below -10, was when they'd not been used all season (so there was no rutting) and they'd been protected from icing by a fairly thick covering of snow (which could be cleared from the lines but left on the rest of the pitch).
When considering whether they've got the facilities for visitors, one of the first things that Butlin's have to consider is whether they're able to clear tons of snow from their car parks, and leave them (and the paths around their complexes) safe enough for people to walk on). If they can't ensure the safety of their guests, they're obliged to prevent them from entering their sites.