It is (for all practical purposes) hard coded into the DOCSIS technology that cable internet uses, as a roughly "10 down to 1 up" use of total bandwidth, and VM can only operate using industry standards, with the most modest of variations. You might expect that would still give you 100 Mbps up, but there's a range of other reasons why that isn't the case - put simply there's insufficient total upstream capacity to give every customer absolutely 1:10 at the contention levels VM operate at, and allowing gigabit customers 100 Mbps ( or 600 customers 60 up) would potentially have a very disproportionate negative impact on all customers. You might even have seen something similar earlier this year, when VM were under the cosh because a lot of the network couldn't cope with the amount of upstream traffic from home working.
It is this way because when somebody came up with the idea of internet over cable, the primary need was download, and the priority was increasing that rather than making more room for uploads that weren't then a big element of traffic. In future there is the possibility that "Full Duplex Docsis" (FDD) could be rolled out, enabling the same speed up and down, but that would require new hubs, and the upgrade of just about every piece of kit VM operate on their network. FDD wouldn't be rolled out unless as part of DOCSIS 4, that can't happen until they've finished the roll out of DOCSIS 3.1 later this year, and even then they'll need to finish amortising the D3.1 investments before a further multi-billion pound upgrade, so a national DOCSIS 4 upgrade won't start before 2026. There's reason to believe it will be subject to secret technical trials even as we speak, but VM don't have to adopt every part of DOCSIS 4, so they might choose to leave out full duplex.
