The 500/600ft figures have appeared in various CAA advisory flyers, in the IMC Rating written exam (very naughty as they are not in the ANO and therefore not
law) and in the fullness of time they have passed into aviation folklore, alongside not leaning below 3000ft, GPS being prohibited for primary nav, GPS being prohibited full stop, GPS not working due to "terrain shielding" ("Safety" Sense Leaflet #25) etc
The CAA issue various bits of paper; for example they tell you to add 1/3 to the POH takeoff roll if it is grass, another 1/3 if the grass is wet, etc. Not wholly unreasonable stuff, for certain types of landing gear anyway, but not
law and that is a crucial distinction.