I just noticed this deal on GAME that gives 10% off the price for Google Play cards, so:
£10 card = £9
£25 card = £22.50
£50 card = £45
Terms & Conditions: Get 10% off Google Play cards in the selection below. Discount is already included in the price. Offer valid until 23:59 on 11/12/14 or while stocks last. GAME reserves the right to amend or cancel this offer at any time without prior notice. The offer has no cash value and cannot be exchanged for cash. This does not affect your statutory rights.
Also worth pointing out the cash back potential on this one:
E.g. a £10 card purchased via Quidco should come in at about £8.75