Hatfield Forest is managed by the National Trust, and there are several areas you could use, as needed. You are not expected to budget for hiring any space within Hatfield Forest, as this will be provided as part of the support from the National Trust.
Outdoor spaces: key touchpoints could be the Lake Area where there are a number of facilities); the Dog Free Zone (adjacent to lake area and near facilities); and Elgins car park (which is a little further away from facilities).
Indoor spaces: Outdoor classroom (near Dog Free Zone above); Classroom (in Lake area); Estate Office mess room (in the NE corner of the Forest, closest to the bus stop opposite the Green Man on Takeley Street)
Shelters: There are a number of gazebos available in various sizes if you need them.
Local village halls
The National Trust has contacts for a number of the local villages and their spaces, but you will need to budget for the use of these:
Great Hallingbury Village Hall
Takeley Village Hall
Takeley Old School
Hatfield Broad Oak Village Hall
Hatfield Heath Village Hall