With the exception of those on Talk Talk, Sky and some Zen customers, all fibre services at the moment use BT Wholesale. BT Wholesale have some congestion issues on their network, in some cases fixable via configuration changes but in some cases requiring new hardware and fibre.
Think Broadband Story
RevK of Andrews and Arnold
AAISP Congestion Report
Plusnet Congestion Report
These issues are somewhat inevitable given increased service speeds, but for BTWholesale are disappointing as these guys make their money selling bandwidth to ISPs, so it's pretty ridiculous them not upgrading their network to keep up with demand from their customers given they're paying per Mbit.
The indignation from Neil McRae was a bit of a PR disaster too.