If you remove a range target without permission, assuming you even get away with it at the time, that is theft and you can expect to get in trouble!
It is much harder to get access let alone permission to remove anything nowadays for a number of reasons. Not immpossible, just harder. There is a survey of all range targets in the UK being undertaken, as I understand it and this will make it next to immpossible to get anything.
Of course, there are old targets that no longer feature and these will possibly 'get out', time will tell!
Some range officers are more amenable than others, I've had dealings with both ends of the spectrum....