I was also doing a nested TP run to TLL today and have been affected by this delay.
I managed to get the AY agent to rebook me on the later flight from TLL back this evening, which was on a separate booking, but will have to stop overnight in HEL as a result, but have A350 on the HEL to LHR tomorrow
Did you book with AY or BA, AY in the past have declined to touch BA's bookings for me.