Data usage on 48 is never charged from your cash credit balance.
It always comes from your membership or add-on data allowance.
If topping up your cash credit balance helped then I'm not sure why.
There's a possibility that your iPhone needed (re?)activation and that needs cash credit to cover the cost of the UK SMS that gets sent.
But if it needed activation then not only data but calls and texts would not have been working.
What was your cash credit balance after topping up and what is it now?