There are 2 codes needed, as VBB indicated. One for visitors and one for Guardians. If (and I don't know) NT authorised the horse thing it's good evidence a Guardians one is needed.
The NT have a statement on Britarch that they were unaware of this intervention, but they cannot side-step this that easily. They took money from Big Brother when the television 'reality' show placed their eye symbol alongside the Uffington White Horse, arguing that it could have been done using photo software and they wouldn't have
been paid a penny - well the payment for that includes everyone thinking that it is acceptable to treat a monument in this way.