IANAL but in their own summary of their trade-in terms and conditions they state
Apple or Vendor may refuse, cancel, or limit the quantity of any trade-in transaction for any reason.
so afaik, wouldn't/couldn't that be read as what's called rescission? especially in good-will on their part within minutes of the agreement being signed? there's no logic in any possible way one could try to scam them that I could think of by doing this with a £0 valuation.