I am from overseas, would the Soil Association be able to sponsor me in the event I do get a placement? (i.e. for the visa requirement, not monetarily
You need to be eligible to work in the UK to join the scheme. You are employed by the host farm, not the Soil Association. Sponsorship application is a lengthy and costly process and it is unlikely that a farm will be able to sponsor you. If in doubt please contact the farm and ask them directly.