Unless you want to know your mobile phone number before you to a country, there isn't any point in buying a PAYG SIM in the UK for that country. In OZ a vodaphone PAYG cost $10 and you then register using your phone which cost a further $10. You then recive $20 credit. So the SIM is free.  In Thailand they are VERY inexpensive and avalable in most shops.  Your phone must be unlocked before it will work. You can get your phone unlock in most countries if you for get to do it here.