I queried this with vodafone and there reply ref the text message is:

I certainly understand how important it is for you to receive the update of the current status of your account.

I would like to tell you that you will receive a text alert at 150%, 200% and 250% of your data usage to you that you've exceeded your allowance.
And price plan / bundle with a higher data allowance might be more suitable.
If you continue to break their allowance we’ll let you know that we’re monitoring your data usage and ask you to choose one of the following options:
Upgrade to a higher data bundle or Upgrade to a tethering bundle.

If you're persistently exceed your allowance will be alerted by the credit team that you’ll need to pay over next month at published rates of::

£5 per 500mb for bundle customers

50p per 25MB for non-bundle customers

Trust the above information helps.