The most likely cause of the issue you're seeing will be peak usage at a local (exchange) level, potentially affecting any non-LLU ISP, resulting in you experience the effects of contention. Obviously the Royal Wedding isn't a factor now, but a couple of weeks ago it will have been. The timing of that event, Zen-wise, would correspond with our network's quieter times (evenings/weekends/bank holidays, as we're predominently a business ISP), however they are consumer peaks so the exchange (BT) back haul will be more likely to experience contention.
BT have planning limits around 'acceptable' levels of contention, and unfortunately the speeds you're seeing would not be sufficient for BT to consider it time to increase capacity. We do detail the thresholds for performance on our website here:
Understanding broadband speeds
I'm sorry you believe we're deliberately taking action to somehow restrict your speeds, but I assure you that's not the case.
ZeN Performance and Process Improvement Manager
15.4" MacBook Pro with Thunderbolt- raaa!
Please note, I will not respond to unsolicited private messages.