[homesecurity]

Your DOCSIS RF signal levels are spot on.
Downstream power sitting between +4.0 dBmV and +5.8 dBmV with an SNR averaging 40.3 dB across all 24 channels (well above the 34.5 dB floor), and upstream power locked between 42.0 and 43.0 dBmV on 4 ATDMA channels is virtually text-book Virgin Media street tap specification. There is zero RF fault on the coaxial or cabinet side.

The bottlenecks you are fighting are entirely within the internal physical topology:

1. The PG-9072LG-VM Booster
This is an older Comtrend HomePlug AV2 powerline unit with a basic 2.4GHz 802.11n (Wi-Fi 4) radio. In an urban area with neighbouring RF congestion, realistic TCP throughput on 2.4GHz 802.11n rarely tops 25-35 Mbps due to beacon overhead and 20MHz co-channel contention. That explains why your speed tests hit an absolute ceiling at ~30 Mbps on that unit.

2. Separate Consumer Units (Electrical Split)
You identified the primary culprit yourself: the ground floor and 1st floor running on separate distribution boards. HomePlug / Powerline signals rely on high-frequency RF coupled onto the copper mains wiring. Crossing through separate RCDs, RCBOs, or different sub-mains introduces severe signal attenuation (-40dB or more) across the magnetic coils and busbars. When the Powerline PHY rate collapses, packets are dropped and retransmitted continuously—which is precisely why your connection drops to 3 Mbps and ping times jump from 20ms to 150-200ms.

3. Bufferbloat on a 50Mbps / 5Mbps Pipe
With multiple housemates sharing a 50Mbps down / 5Mbps up tier, whenever someone uploads a file, backs up photos, or streams high bitrate video, the 5Mbps upstream pipe saturates. When the upstream buffer fills up, your outgoing TCP ACK packets get delayed in the router queue, causing the exact periodic latency spikes you are seeing in your ping logs.

Best options as a tenant:

- The Cleanest Fix (Wired Backhaul): Powerline simply will not work reliably across split fuse boards. If you can't drill walls, run a flat, discrete white Cat6 cable (external grade clipped along the outside masonry, or run neatly along the skirting / under carpet edges) from the router up to your 1st floor room, and plug in a cheap modern Wi-Fi 6 access point or gigabit switch. That will deliver your full 50Mbps line rate with a rock-solid 1-2ms internal ping.

- Wireless Mesh (Bypassing Powerline): If physical cabling is completely out of the question, return the VM powerline plugs and look at a dual or tri-band mesh system (like a TP-Link Deco pair). Place the secondary node in the hallway or at the top of the stairs where it has direct line-of-sight down the stairwell to the main router. This routes the 5GHz signal through open air rather than trying to punch through solid interior masonry walls.