I had this problem a year or 2 back with Demon, they were using some Tata owned company for peering and the atlantic hop was always getting conjested then slowing down, then losing packets. even when the hop across the pond was ok there were delays in NY between Tata and AT&T. plus Demon's internal routing was becoming rubbish - too many hops and packet loss. after years of reliable connection it suddenly became pretty much unusable.
thats why I switched to Entanet (via one of their resellers Aquiss) because they actually seem to care about getting their peering right (enough capacity) and have the techies around to run their network properly, although that does cost more. last night's ping was pretty high for me, its usually ~120 with 14 hops.
We have 2x ADSL2+ lines at work for bandwidth sharing, one with Demon and one with Aquiss for redundancy:
Demon averages 4.5Mbit rx and 480Kbit tx
Aquiss averages 8.5Mbit rx and 950Kbit tx
... using the same modems, to the same exchange, in the same multicore cable ...
bottom line is not all ISPs are equal.