In terms of the big retail ISP's, I have moved about a fair bit, mainly to get cashback deals.
I have been with TalkTalk, PlusNet, BT and EE.
I have never experienced speed or ping issues on any of them. I had a lengthy outage on TalkTalk due to a fiber break but that was it. I used my own kit though, which I do now too,
Ping times for me were the lowest on BT - a couple of ms lower than TT.
Congestion - none. Always had full speeds at all times.
Support? I spoke to TalkTalk and BT via their support forums and they were very helpful.
I did briefly use BT's kit before I bought my router and it restarted every 14 days, which annoyed me.
I left BT because they weren't particularly competitive when it came to the renewal. I'd have stayed otherwise.
I see Uno have been mentioned, brilliant ISP. I was with them on ADSL for a few years and Matt was superb. The customer support and connection were of high quality. I would definitely rejoin them in the future - on BTw backhaul though.
We have had a couple of outages in the town recently that have wiped out Sky and TT on all the exchanges yet BTw connections have been fine.
EE Fibre Plus 79690 | 19999kbps