I'm not very far from you and this mirrors my experience. In fact you get a little more than me most evenings it seems, and I am on the 200 Gamer package.
The issue we both face is primarily due to what is called 'over-utilisation', although compared to complaints generally made on this forum by some members, we are both relatively lucky. Some receive far less than 10Mbps! In essence, everyone comes home in the evening, switches on their computers and TVs, and the shared bandwidth available quickly gets saturated. This results in the drop between your expected max and the average bandwidth actually available in the area. You need to remember that cable is actually a shared medium.
The good news is that 30Mbps is still pretty decent. Most of us also in our area don't have BT to fall back upon as most of central London (contrary to popular opinion) is not covered by FTTC/FTTH/FTTP. 30Mbps therefore is still better than anything that BT is willing to provide. Also it is more than fast enough for most services such as Netflix and Amazon Prime.
This is probably not what you want to hear though. The only way things will improve is for Virgin to provide more backhaul capacity. Sadly the ducts in our area are fairly full, more bandwidth costs money, and as we are a captive audience..........
FWIW, I wrote a letter to the BT CEO and Chairman a few years back re getting FTTC/FTTH/FTTP and was told to request funding from the rural broadband fund. Needless to say, I know that is not actually a rationale response as it was not designed to fund internet connectivity for central London residents, so we really are just left with Virgin.
Your other realistic option is Relish (UK Broadband, although recent taken over by 3) which provides an LTE based service to central London residence. Sadly most of the reviews of the Relish service are pretty poor which is a shame. Having said that, as they used LTE in the 3.5GHz part of the spectrum, building penetration is not great and therefore none of this is really surprising. If you can arrange for a shared antenna for your building, the connection issue can potentially be gotten around, although you will still be limited by speeds they currently provide residential users which I believe tops out at 40Mbps.
I'm really sorry to hear you're having trouble with your speed.
I've located your account and I can see an issue with the network in your area affecting speeds at peak times. I've arranged for one of my colleagues to contact you within the next few days to give you more information.
All the best!
Tech fan? Have you read our Digital life blog yet? Check it out