If you want to add extra credit to your UK sim card, you'll need to wait until you arrive in the UK and your sim card is activated.
To add extra credit once you get to the UK, you can purchase extra credit from us, or you can purchase a Three UK top-up voucher from any Three Store or from any shop in the UK where you see the green Top-Up logo (such as convenience stores). Then just call 444 from your phone and select "Top Up By Voucher" and enter the 16-digit voucher code.
We sell extra credit in the following denominations: £10, £20 or £50. If you would like to add extra credit, please email us at info@ukprepaidsimcard.com.au - you can purchase the extra credit from us now (it comes in the form of a redeemable voucher code - you redeem it by calling a phone number and entering the voucher code), but you won't be able to actually redeem the voucher until you get to the UK and you've activated your sim card.