Google finally launch Google Play gift cards in United Kingdom. You can find in Google Play Support page listing as available in £10, £25 or £50 denominations. If you sign in to your Google Wallet and find your Google Play balance is low then you can recharge your balance with these Gift Card.
In earlier August Google launched the Play gift cards in US which are available in $10, $15, $25 & $50 USD and now it comes in UK. The Google Play Support page also goes on to says Google Play balance has a maximum limit that may differ depending on your location. For United States it stand on $2,000 and £2,000 for United Kingdom. The gift cards may be available in the shops across the UK, Carphone Warehouse and other related places. And you can also check on this UK page for all about Gift Cards.
Source: Google Play Support