For only 12 kg of catfood (@£55.96) you would earn enough points for a free return ticket on the Heathrow, Gatwick or Stansted Express, plus have leftover the 110 points one would normally earn on such a purchase (and another 27 points if one used a Barclaycard to make said purchase). If you actually might need that much catfood, that could be a good deal, albeit not an arbitrage opportunity.