Openreach supply the FTTC equipment, not BT, there is no way , or any reason for ‘BT’ to reduce your speed , a line problem could cause OR’s dynamic line management (DLM) to intervene, sacrificing some speed for stability, but without your router stats it’s not possible to know.
If you switched to VM, you would be on a separate network to any provider that uses OR ( BT, Sky TT etc ) but Sky ( for example ) would , if you purchased the same type of product , would deliver approximately the same speed.
If you checked at the test port with a corded phone ( cordless phones are designed to remove ‘noise’, so the quiet line test not as effective) and there was no noise , perhaps your line is ‘banded’ , this ( providing what ever line issue caused it is no longer present ) should eventually be removed...just because nothing has changed internally doesn’t mean your internal setup isn’t above suspicion, and if you haven’t already tried it, you should try your router in the master socket test port to eliminate any internal wiring/sockets.
As already stated , more people sharing the available bandwidth doesn’t affect your connection speed, and if congestion were an issue , generally you would only see the ‘throughput’ issue at peak times , if you are considering VM, a little research on over optimisation on VM cannot hurt.
Finally , you could be suffering as a consequence of a REIN/SHINE ,you may want to search on what can cause this
