Northbound out of Euston, there will be two lounge cars in the formation, one for the Aberdeen/Fort William passengers and another for the Inverness passengers. If your lounge car (the Aberdeen/Fort William) has failed, you should be able to go down to the buffet in the Inverness lounge car to order food to take away. The staff may well let you sit in it, although bear in mind that it may be restricted to first class passengers because of the need to cope with two cars’ worth of customers. However, if both lounge cars are gubbed, then there will be no catering at all in the evening. The attendants’ pantries only stock pre-ordered breakfast supplies, and cannot (generally) provide any catering other than pre-ordered breakfasts in the morning.
If you’re going to Fort William, bear in mind that a new lounge car is added at Edinburgh to your section so this should be working for breakfast/morning tea (but note that the reliability of these Mk2 lounges at the moment is not great, so this could be gubbed too). You may want to think of contingency plans in case lounge cars are out of action.