Theoretically three UK is best. For 30 days you can enjoy 12GB of data for 20 GBP in Australia, Austria, Denmark, Finland, France, Hong Kong, Indonesia, Israel, Italy, Macau, New Zealand, Norway, Republic of Ireland, Spain, Sri Lanka, Sweden, Switzerland, USA. If you are not in the UK, http://www.ebay.com/itm/191861285481 these guys will gladly ship you a SIM and once you get the SIM they will activate and load the 20GBP on it. Once you are ready to use it, you can buy the All-in-one addon.

One disadvantage: the All- in-One £20 plan gives you lots of free minutes and texts but only to the UK. If you need to call or text some other country, use VoIP.

Disadvantage number two: some people report crazy throttling.