Portable goals in the UK MUST be held down. Covered by point 1 of the linked document, which isn't just from the FA but also the Departure for Culture, Media and Sport, Health & Safetry Executive, and the British Standards Institute. Any referee not enforcing this, and should someone be injured, could easily find themselves liable to serious problems.
http://www.thefa.com/~/media/files/...-association/2015-16/041_goalpost-safety.ashx
The only exception is where the have a weighted back bar, so that is where the bar at the back of the goal is heavily weighted to make it impossible for the goal to topple. These usually have some kind of indication on them to state they are weighted.
Where not weighted they should be pinned to the floor. On astroturf I've seen this done by fastening it to a pillar on the surrounding fence with a strong chain and padlock, or the back bar weighted down with heavy sandbags..