I cant speak for the business products which obviously will be given priority over home.
My own exchange is (was) notorious for congestion and why I used to pay extra for max premium (20:1) compared to max (50:1) in the IPStream days of old before moving to BE* LLU.
When it comes to FTTC then you will share bandwidth across all ISPs between the cab and the exchange. However that said its unlikely congestion should be an issue at this point. Since 21CN BTw doesnt have the same backhaul issues from exchange to the core network either. I havent heard of any 21CN backhaul isses with congestion at all. With GEA FTTC backhaul depends on the LLU provider so cant speak for those.
I personally stayed away from Sky because several times seen signs of sky backhaul congestion in this area - admittedly it was all on adsl2+ though.
Knowing what this exchange can be like I chose my ISP with trepidation, Zen was a bit expensive for me at the time, although they have since brought their prices down. I went with Plusnet just the normal home, but the unlimited package as I do a heck of a lot of streaming TV.
Even during peak Ive never seen anything less than 70Mbps throughput. This is a speedtest Ive just done now
I believe only p2p is shaped on unlimited, since I dont use p2p Im afraid I cant give you any indication of how that fares. Over the past few weeks Ive been doing a lot of external FTP traffic and that seems to be more or less my line speed. Hard to say for sure because some of the files I was transferring were done in the blink of an eye.
FEXT currently seems to be more of an issue than congestion with FTTC.