Another day another cancellation

This booking was on a companion voucher for a flight to Canada 2 years ago, which then became a FTV. I've used is for a domestic GLA-LHR in J (I know it's not the best use of it, but that's a story for another day)

I was on BA1488, 19th June Dep LHR 16:05, Arr GLA 17:35 (Now cancelled), I've been re booked on BA1496 Dep LHR 21:55, Arr GLA 23:30 (That's the only option showing for me online)

When I log in online, there appears to be availability on the 18:15, 20:00, 21:00 & 21:55, the 21:55 really doesn't suit at all, If I can ever get through on the phone (Tried several times today), can they refuse to move me to one of the earlier flights?

As always, any advice welcomed.