C & W is a wholesaler, as is BT Wholesale.
The difference in user experience wrt throughput and congestion with BT Wholesale ISPs, from poor to excellent, is down basically to three things: the amount of capacity they rent from BT Wholesale to handle the traffic between you and the ISP; the throughput capacity of their own routers; and the capacity they rent from those routers out to the wider internet.
The same will apply to C & W based ISPs.
Just like Sky LLU compared to Sky Connect, O2 LLU (as was) to O2 Access; and TalkTalk, who it seems provide a bare minimum transfer capacity from BT Wholesale for their non-LLU customers, I expect VM only provide a minimum for their DSL ones.
I believe all of them only provide BT Wholesale based services so as to be able to claim national coverage.
I don't think you need worry about most C & W resellers. They live by providing a good customer experience. Have you a preference among them?
Edit - just realised you are looking because of what Xilo/uno have just told you about O2 Wholesale. So I assume Xilo/uno are your preference.
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Edited by RobertoS (Sun 27-Oct-13 14:27:42)