I’d suggest you can breathe a sigh of relief if it is NFU.
I did some work for them as a contract claims inspector and, generally, their philosophy is to look for a reason to pay a claim, not to challenge it. It’s their marketing tag and actual philosophy.
If the farmer is taking money for the parking rather than a gesture of good will, a contract has been established.
He ought to have ensured that cattle could not damage his “customers” vehicles.
You should* be ok.
*it was around six years ago when I last worked on NFU.
Brings back memories when a driver in a BMW crossing some open access grazing land had his bonnet squashed when a horny bull tried to mount a unwilling cow by a road side, fell off and landed on the bonnet!