I'm not sure it is illegal........
If you had the landowners permission to release rabbits why is it illegal ?
They are a native species.
Foxes can legally be released (I checked with DEFRA) but it's not advisable because they can cover huge distances to get back to their home territory and can spread disease in the process and have to contend with other foxes whilst trying to get "home"
Non natives like grey squirrels must be destroyed if trapped.
But how many times have you seen those idiot vet programs setting the fluffy buggers free after nursing them back to health????
Who got prosecuted ? No one.
Do it.