Pace
Ok here's an example. The ash cloud is located from the uk to about 30 west and ETA is 0730 uk. I can assure you that when you first enter the Enhanced Procedures Zone mid atlantic, in this made up example, it sure won't be daylight! Yes the sun may be up in London but abeam Keflavik the most you would probably see is a faint glow in the east.
In fact the ash cloud when this all started actually got about as far east as Gander or thereabouts. Since then it's made it down to the Azores.
The average long haul jet in my company makes a round trip in about 24 hours. In order to avoid flying at night means you would lose one rotation. You might have a point IF the ash cloud was only affecting the UK but unfortunately it's not as easy as that. In any event I do not know if the CAA would allow a see and avoid method to be sanctioned.