I wouldnt join plusnet right now, although their action (eventually) to deal with the congestion is a work in progress, I would wait until things are confirmed to be improved over a period of time.
Also personally I think plusnet are more closed up then sky tech support, and sky is a bigger isp. On the pro section of the sky forums they have been quite open in some requests e.g. detailing capacity of individual exchanges, utilisation levels (rather than a vague colour code) and fix dates, as well as on backhaul and transit routes. Would never get that kind of info from plusnet now.
They also refer to openreach as openreach not "our supplier".
I would recommend either sky or pulse8. Both will avoid BT wholesale assuming in a LLU area and have good feedback from existing customers.
Zen seem to be suffering lately, which could be down to BTw or themselves.