Our EE Internet service (which like many other providers, uses the same Openreach ‘last-mile’ infrastructure from the BT exchange) hasn’t been affected by these issues, consistently gives about 30Mbps (20Mbps to 50Mbps) and is also cheaper than BT.
Perhaps the problem with BT lies elsewhere i.e. within BT’s own network and out through to their Internet gateway? Or maybe their newer style Home Hub Routers or default settings need to be updated and checks done to see if you are experiencing intermittent channel interference from nearby routers/other devices?
If BT are not providing the service you are paying for, you should legally be entitled to cancel the contract and go elsewhere.
You could try getting together and lobbying Virgin Media to expand their existing Business-Only full-fibre service network in Brookmans Park to residential premises, but as mentioned previously, they will probably want the substantial full build costs up-front (well beyond the budget of any single household!).