Upload is pretty much most important and its your data/requests going via it that make downloading happen, for many years its not be considered important but in the last 5 years I've been somewhat advocating its importance as more and more people start streaming and sharing online.
Download now has become so much of a known headline and seller in the market that is more focused upon, everybody seems to be obsessed with the Gigabit (1000mb) headline these days that they overlook the meagre upload speeds supplied in some cases (such as worst case scenario which is Virgin's own Gig1 which currently only capable of 50mbps upload , unless obviously in area with issues like yours)
Unfortunately operators like Virgin do currently have specification limitations holding them back as cable broadband while great for delivering high speeds to customers is not currently very adapted to 'taking' speeds from customers.
The reason for this is that the ratio of download to upload bandwidth on Virgin kits being very lopsided towards delivery of services (download) there really isn't much upload bandwidth to share amongst the local area loops connecting customers properties.
This is isolated to DOCSIS (Cable Networks) only, most other technologies in use to connect customers to the internet is much better managed and designed to lower the ration of customers to bandwidth and also physically limited to accept fixed numbers of connections rather than the rampant splitting that can and does occur across many parts of Virgin's national network.
Recently I run some number to work out how many customer it takes to saturate the available bandwidth on a localized loop of a typical Virgin network setup and this gave me an estimate that it only takes a very small proportion of a an area to bring the network to a virtual halt.
It takes between 130-180 kids learning from home to saturate the upstream bandwidth bu using a google classroom session each, this don't include the parents working from home of video conferences and remote connecting to PC's at the office. this was based on a Virgin network node that served 768 properties of which only 50% had service had service with Virgin and only 65% of them properties had 1 child at home.
As for the blame, you can of course blame Virgin but you won't get much help from them, if you wanted to look at other ISP's then they would let you go based on speed issues without any penalties (If that was a route you would consider), currently the best hope for services resuming some sort of half decent performance is just waiting it out.
I recently dropped one of my Virgin connections in favor of a slower DSL service as the upload was becoming unstable and affecting some applications in my home office, it wasn't anywhere near as bad as your own problems but enough to affect some latency sensitive operations, I went from having 350Mbps download and 36mbps upload to 50mbps down and 19mbps up and I can't say I've noticed losing the bandwidth on download and uploading now is stable like never before, I also share the connection with 2 other adults who stream Sky/BT sports constantly of a night.
As much as a read this is I hope it gives you a good idea of what broadband is about and what is important and not.
The Giggitty 1
