Vodacom
Why does data roaming not work in UK?

I am traveling in the UK and want to access the Internet when I am out of wi-fi range. To that end I activated all the required services before I left SA, but roaming on my cellular connection does not work. The call centre told me that the reason was that I had a top-up contract and not an open contract.

Is that correct? How does the way that I pay for the service affect what features are available to me?