On the outbound you will need to travel from GLA otherwise the rest of your flights will be cancelled. For the return, could you travel HBO? If not, there are two real options. 1) rebook the final LHR - GLA leg to the following day but less than 24 hours later (or the same day from LCY or LGW) 2) On landing at LHR go to baggage services in the baggage hall and explain that you won't be travelling on the final flight due to a last minute change in plans and that you want your bags back. It might take an hour to get your bags back, but you'll get them back.