validation.rb

module ProfileValidator
  def self.validate(profile, scentregroup_id)
    profile_schema.call(profile.merge(scentregroup_id: scentregroup_id))
  end

  def self.profile_schema
    required(:centre).filled
    required(:number_plates).each do
      schema do
        required(:plate).filled
        required(:registered_in).filled
        
        validate(plate_available: %i[plate]) do |plate|
          ### How can I use the centre attribute in here?
        end
      end
    end
  end
end

Ah ok. I am a bit relieved that I was not missing something super obvious, thanks @timriley :)