ASDA Mobile SIM cards can be bought online on their website or in many ASDA supermarkets and grocery stores throughout the UK: store locator for 50p. You can top up at any ASDA store or anywhere you see their logo buying a top-up voucher.his roaming option is available to all pay-as-you-go SIMs of Three UK and is valid in Austria, Australia, Denmark, Finland, France, Hong Kong, Indonesia, Israel, Italy, Macau, New Zealand, Norway. Ireland, Spain, Sri Lanka, Sweden, Switzerland and in the US (on both AT&T and T-Mobile networks!!).