Openreach's solution for the upstream uses a totally different frequency plan. Someone can correct me but the upstream and downstream are in the same frequency zone somewhere like DOCSIS downstream. So upstream isn't as susceptible to RF noise.
For VM to improve upstream/downstream symmetry, and given that they cannot leave the specturm they're using on DOCSIS 3.0, bonding is the only uplift solution.
I'm not privy to VM's DOCSIS 3.1 plans. But a layman's description of what needs to be done fore the upstream can be found here:
http://www.lightreading.com/blog.asp...27176&site=cdn
This would put the upstream into half the space currently occupied by downstream, which would move further up the frequency spectrum. However, this would still not provide the same number of total channels for the upstream as there are for downstream. So bonding would be lower; QAM would be higher than at present and upstream speeds will remain asymmetric with downstream but will be higher than at present.