With charges like that it kind of works like the following: if you access the data side outwith your allowance or when you have no allowance period, it would charge a flat fee for the entire day. O2 and other providers does this too. So it's not £2 for 10Kb, but £2 because you accessed the data network outwith your allowance and a charge was applied to give you 24 hours access to data.
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Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.