Slug pellets can no longer be sold or used in the UK, as of Friday 1st April 2022, as they pose an unacceptable risk to birds, dogs and mammals. This is specific to those containing metaldehyde.
Slugs and snails are now no longer classified as pests. Organic and wildlife friendly methods of protecting plants are recommended. Slug killers based on ferric phosphate are an option if used sparingly and stored safely. They contain ferric phosphate or iron III phosphate, which affects the gut system of snails and slugs causing them to stop feeding and die within three to six days.
Overall though, it would seem sensible to refuse any offers or requests of slug pellets on Freegle.