Regarding Jack, I found out yesterday afternoon that he wasn't able to come because of a personal connection I have to the film. Jack is not able to come because he's got to have heart surgery. He's devastated at not being able to come and would do anything to be there, but his health has to come first.
The Superman group photo was a big deal because this was a major thing to get Terence Stamp to a convention for the first - probably last - time. The hype was to have the super villains reunited for the first time since Superman II was released and was the major, though unspoken, selling point of the shoot, hence the high price of £110. It is unreasonable to expect the event to not offer a refund of some sort because, in reality, it's sold at a high interest for that reason. As such, if they don't offer a gesture of a discount, it would be both unreasonable and generate a LOT of complaints from the people - including myself - who purposely booked the shoot for that reason. And that reason alone.
If there is not the choice to either refund and cancel or refund partially, because it stated all along - Terence, Sarah, Jack & Margot, that is what is expected, then legally speaking, you are entitled to demand and receive a refund because that is not being delivered.