Found this on the T-Mobile website:
Mobile Broadband - RoamingIf you use mobile broadband or web'n'walk in any of the following EU countries it will cost £1.50 per MB you use, and you'll be charged via your T-Mobile bill. The EU countries included are: Austria, Azores, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Madeira, Malta, Monaco, Netherlands, , Poland, Portugal, Romania, , Slovakia, Slovenia, Spain andSweden. If you use it in any other country, it'll cost £7.50 per MB. We're afraid that roaming isn't available for Mobile Broadband Plus Daily customers. When you start roaming you may be asked to pay a deposit.