I think the PON is the main driver behind the 110Mbps on FTTP from Openreach, and it's hard to see how the PONs would get saturated currently from such a TV event.
Even if there were 30 households on a PON, 2 screens per house, generously 20mbit/stream - =40mbit * 30 = 1200, still enough room for at least one full rate speedtest.
I think there is more likely to be an issue upstream.