I am not on FTTC, I have moved to FTTP and when I was on FTTC the influx had no impact on my speeds which were 17mbs all the time, anytime.
I was told clearly that I should expect little if no slow down from 300mbps and 50mbps and right now I am averaging low 100's daytime and low single figures in the evening. It is their problem and they will I am sure fix it and make good for my time and costs as they have already offered to do.
These things happen, annoying but not something I get angry over......well not yet anyway.