Sorry your holiday was ruined - i can't understand the crew being out of hours either.
I know it's no comfort but the staff at LGW are not VS, they are an agency and a fantastic example of why the company should have it's own staff. They failed passengers and the company.
I work at LHR and even though i've never done it live, i could quite easily do manual check in. It is easier and in fact offers a better service especially in terms of seating as only a limited number of agents have access to the seat map.
If organised properly, manual check in shouldn't cause a flight to be in that much disarray.
Sure we have to write boarding cards out and then get them validated (as any one could write one out = big security risk!) but it can be done effeciently.