You are correct that you only get 1 guest. LHR-ATH I believe is T5, so you won't be able to use the AC in T3 where you can guest people in. There are some priority pass lounges in T5, so that might be an option to split up or pay for extra guests there.

On the way back. you can guest 1 more as your daughter has access by her being in business. That will allow 3 of you in to the lounge.