By Alistair Smout LONDON (Reuters) – Cancelled flights, long queues and staff shortages have disrupted the travel plans of British holidaymakers during the half-term break, a grim sign of possible summer strife for a global sector battling to rebound from a two-year COVID slump. The next few months will be the first time since 2019 that the aviation sector will be able to enjoy a peak summer season free of substantial coronavirus restrictions, but staff shortages at airlines and airports may hinder that outlook. Scenes of chaos have been seen at some European airports over the last week as peo…