You may have been caught out by the unhappy meshing of System Internationale scientific units and binary based computing terminology.
In SI nomenclature, a capital M is always "mega....", usually meaning 10^6, or 1 million of whatever. In computing that can be a bit of a flexible friend, because a kilobyte is 1024 bytes, creating the option that "mega " might mean 1,000,000, or 1,024,000, or 1,048,576, depending on the intentions of whoever is using the numbers. Meanwhile, a lower case m refers to a "milli" meaning 1/1000 of whatever is in question. Meanwhile, a byte is always a capital B, and a bit is always a lower case b.
So strictly speaking, mb would refer to 0.001 of a bit. 1 Mb would be 1 million bits. But although there's eight bits to a byte, that doesn't account for start and stop bits, which have no information value, but may mean that you need ten bits for a byte of "payload", so I'll leave you to work out how many bits there are in a Virgin Media mega byte, and what that means for the advert.
But if you think you're confused, imagine those ideas rattling around in the vacuous, empty humanities graduate skull of a marketing employee. It'd be like Matt Hancock trying to work out anything involving numbers.
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