Just to close off this post, I have identified the issue, and it is now fixed. However, I do have some feedback with the Superhub 3 that I would appreciate being passed on to the correct people at Virgin Media.
The Issue:
Ever since I've had this new hub (previously Superhub 2 AC) I've been experiencing issues with the wifi. The internet works perfectly through LAN, and I've not noticed any issues there. However, when using the wifi, my devices will connect fine, but then after a while the internet will not work. (See in original post all the potential reasons this fault was happening, and the things I eliminated as not the cause.)
The Fix:
The fault was found to be potentially faulty ethernet cable. This took months to identify as the cause because there were no signs of it being faulty.
I found the fault be installing a new router and putting the Superhub 3 into modem mode. That router worked fine, except when this particular ethernet cable was plugged in. I found the fault just by accident. When the cable was not plugged in, all wifi and internet services worked perfectly. Doing speed tests showed that the download/upload speed was high, and the latency was low. However, as soon as I plugged in the cable and performed the tests again the speeds dropped by 50mb and the latency doubled. The wifi also started to perform poorly when that happened.
I repeated the test several times to be 100% sure. Changing variables such as no ethernet cables, then testing with 1 plugged in, then plugging more in 1 at a time. Even testing to see if it mattered whether it was plugged into a particular ethernet port, and whether switches on the network had anything to do with it.
It eventually came down to it being that cable. Which was odd because I was able to access my server on the internal network and access the internet fine through that cable.
I have no idea why this cable was faulty, or what the fault was. I even checked the ends and tested it with my tester, no issues. The funny thing was I had another cable of this brand laying around, this one was only 0.5m but also had issues with the network! So it may be the brand.
The brand is Duronic. Here it is on Amazon: https://www.amazon.co.uk/Duronic-CAT6a-Professional-Shielded-Network/dp/B003CL71II/ref=sr_1_3?ie=UTF...
I hope this helps anyone in the future with an issue like this.
PS:
Superhub 3 Feedback
You guys really need to work on getting the administration site (192.168.0.1) sorted out. It’s so slow!
The router does seem to be a little flaky with mobile phones and tablets on wifi.