I'm no expert but I think you should be able to add the flight to your itinerary.

However, even with separate tickets you would be able to check your bags all the way through and with OLCI you wouldn't have to check in again at LHR anyway. You could certainly stay with you mum all the way, so it may not be worth doing.

The only advantage of getting it all on the same booking is that you would be protected if the GLA-LHR flight were delayed.