Quote from a post by Edward I've had some discussions with a very nice person at the Environment Agency.
Their response is very clear and helpful. Briefly (in my words):
The issue here is entirely for members, who may be obliged to register, most commonly if they are doing this as part of their business.
See http://www.environment-agency.gov.uk/business/sectors/wastecarriers.aspx for more details.
More generally, everyone using Freegle is responsible for their Duty of Care to dispose of waste responsibly.
Local Freegle-affiliated groups, and Freegle nationally, are under no legal obligation
to register themselves,
to verify the registration status of members,
to ensure that members are familiar with their obligations under the Duty of Care.
So, you can allow scrap metal posts if you want, and you might choose to point out to people asking for scrap that they may need to register,
but you are also fine not worrying about it at all.