(entirely speculative) They may be doing it precisely for that purpose. Risk systems generally working on scoring mechanism, if the £1 transaction was enough

(entirely speculative) They may be doing it precisely for that purpose. Risk systems generally working on scoring mechanism, if the £1 transaction was enough to push you over the threshold then your cards may well be on the borderline for "this card might be stolen". Google might be using it to test if the card is a borderline "stolen card" case because they want to be extra cautious about who they ship phones to.