The 'law' governing flights in ash conditions is the ICAO document referenced in previous posts. Who exactly could be sued and for what reason?
Well, yes and no. ICAO EUR Doc 019, if that is what you are referring to, comprises guidance and recommendations for various bodies to follow if a volcano blows in the N Atlantic area, but is a very long way from being law in England and Wales or anywhere else in the UK.
Any other ICAO document would have a similar status.
IF the person or organisation who decided on the airspace restrictions got it wrong, or was not even empowered to do that in law even though he/she/it obviously can in practice, repeat IF, then that's where the airline lawyers will aim.
Reason? Don't be naive; money, and lots of it, in compensation at taxpayer expense.