Hi Ian,
Sorry to hear of the lousy predicament your in-laws are in.
Short of what others have already said, BT aren't going to be liable for anything other than repairing and/or replacing the master phone sockets in the homes of the affected people/families. Basically, BT will claim that lightning is an "act of God", and as such, they aren't liable. It's a fairly common clause in most contracts to do with home insurance, and companies like BT who supply homes with certain items. Ditto with gas and electric companies too.
Alas, you have little to fall back on that your in-laws can use as ammo against BT! You can try to get in contact with the manager of BT (whoever he/she is), but your complaints are likely to fall on deaf ears.
If you're very nice, you could ask Sky to see if they would consider replacing the Digiboxes for new ones, and put a spin on your in-laws age (e.g. it's Gran's only source of entertainment in life, she's housebound, and she's so frail she could pass away any time now...., etc - my apologies if that sounds cruel) and see if that works.
Otherwise, it's down to regular household insurance.
Sorry to be the bearer of bad news. Legally, I'm afraid you're f***ed! They don't BT the Bloody Tightwad company!
Pooch