Most trespass is not an indictable offence, criminal damage* is. With agricultural crime being what it is at the moment the actual arrest (even with tying up suspects if one feared for personal safety) is likely to be a fine split on whether the farmer would be charged and convicted. More likely is a charge relating to the unsafe transport of the prisoners.
*riding an electric bike might cause criminal damage to arable land, if there was livestock then that gets more complicated.
Citizen’s arrest relevant law
