"Switching from one lousy OpenReach reseller to another is not going to improve service."
Actually, it can. A lot of the big providers over-sell capacity and thus you get congestion and slowdowns. It's only the last mile that is openreach, beyond that it's down to your supplier and if your supplier is stuffing 5000 customers down a 100Mb circuit to the internet at large, you're going to see some congestion.
The smaller providers generally don't try to maximise profits quite the same.