any decent mechanic would know that valve damage could occur if the camshaft breaks and i reckon they should have at least warned you this could have happened and how much it would be to replace, replacing the valves is a timely job, as they have to be ground in (my sons a mechanic and he did his own when his timing belt snapped and damaged some but not all of the valves on his clio) took him 3 days to do the job at home as he was working full time and then doing it after work.
as you say you feel misinformed about the situation, im really surprised they would rebuild the head and camshaft knowing the valves were damaged, im not sure that you have recourse to blame the garage you say had they done that you would have scrapped the car, but for them to really know if any damage has occured to the vales they would have to strip it down anyhow, with no compression in the piston chambers the car is never going to fire, unless the valves are changed and re ground in. i think if they had told you you would have had the option to scrap the car before being handed a bill for �250