Canadian iPhone 5 in UK

Hello,

I bought an iPhone 5 from the Canadian Apple store a few months back. I have just got back to the UK and inserted my UK sim card. I'm on Giff Gaff (O2)

I can still make and recieve calls and texts. But most of my apps aren't working. Nor are any of my browsers. iMessage still works over 3G so i dont understand why my apps and safari arent sending or redceiveing data.

Does anyone have any advice?

Thanks in advance,

Tom