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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By convention we almost always use bits per second (bps) for download and upload speeds whereas bytes is used for storage capacity. So, it's correct to say that a 5G phone has a 140Mbps speed (and as it happens it's a respectable speed) but 'megaBITS (mb)' is confusing for the uninitiated and there is never any need to use bytes per second (just because you can is no excuse for doing so when bps is the convention).