Well I'm back home and on expertflyer so thank you everyone for the help. They're currently in the Q at Gla to talk to the BA desk. I've told them to push for the VS on Sat night and get on a train down to London but whether GLA will be able to rebook them or it's a phone call I'm not sure. It sounds like when the flight was cancelled there was no BA ground staff in GLA (appreciate it's all contracted out).
Aaaand when they got to the desk they've been told they've been rebooked by BA ex GLA on 5 July and the desk can't do anything about it. 🤦♂️