My speed dropped when they enabled fibre on my exchange as well. Then I noticed my line length had increased by 500 meteres. I don't know what happens at the exchange when they start work but I'm sure it's a major job getting fibre installed, some lines need to be rerouted, some need to be consolidated into larger groups. It's just a symptom of the upgrade.
Your line might have been happily running on it's own line card and now you're sharing with a larger group so they can make space for the extra equipment.
Who knows!
If you still think there is an issues then check for a potential line fault. Call the desk on 0800 800 154 and go through the options for broadband fault.
Thanks