I think it depends on your background and skills - if you have experience in the field or are a teacher seeking a career change i would think it would be fairly easy just a matter of approaching prospective employers and asking - best time to ask being at the start of the summer term as this is when school visists are at their peak.
if you dont have the experience then you probably need to volunteer first to get some - check out the major environment and conservation job sites such as
Environmental Jobs and Volunteering in the UK and
Countryside Jobs Service - welcome. for voluntary opportunities
btw is there a particular reason why you want to be freelance ? if you are just seeking flexiblity in hours you will find that many wildlife trusts and study centres are preared to be quite flexible if you are good at what you do.