The obvious question would be whether they would have sufficient traincrew to operate additional services?
(Train drivers take holidays too ...)
Indeed, in a lot of industries it's quite common for there to be a requirement for annual leave to be taken at times of lower demand, or could freight drivers operate the 91s if there is less freight work over the summer? I doubt many sound or lighting techs in Edinburgh are on holiday at the moment, retail workers tend not to be allowed to take much leave around Christmas... There is also a general question (though I don't think GNER are badly affected as I guess as a 'glamour' TOC, they can poach drivers from other companies, of many TOCs operating below the appropriate number of drivers as overtime is cheaper than training more staff.
Still, it's happening in 3 days, I feel most sorry for the likely poorly paid staff at Kings Cross and Waverley over the weekend who're going to be trying to redirect a lot of passengers.