An obscene sum, like the amount that painting fetched.
I agree.
Why not have 184 people win £1 million each?
The answer must be that people no longer consider £1 million enough to get excited about, or at least that lottery organisers think that is the case.
I would keep (say) £14 million to spend on comfortable homes for myself and my nearest and dearest, money put to one side for maintenance on those properties, and invest enough for any normal future care needs for all of us. The younger ones could then choose jobs that they liked without having to worry too much about how much they were paid. The older ones could retire in comfort without any financial worries.
I would enjoy working out exactly what good causes to invest the other £170 million in!