Texts / SMS to Australia are not included with the UK Phone Sim Card.
Texting to Australian mobiles costs 35p/text and requires you to add extra credit - we suggest our customers just use Viber, Whatsapp or iMessage to text Australia.
To add extra credit once you get to the UK, just 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.