Thanks so much for everyone's opinions. They've been a great help.
Basically, the holiday is already booked (dates are fixed) and flights are fixed with Monarch. However, they have the option of changing the flights if they wish.
Various factors such as price mean that a premium cabin won't be an option, and carriers such as Swiss, Singapore and Qatar don't offer particularly helpful connections on the travel dates... from what I can see.
Armed with your information, I've suggested they look into EK and BA with the agent to see how much extra they'll be as the possible LGW connection issues with Monarch and BA are potentially a problem. Have also suggested they check out the legroom upgrade on Monarch if a change to E/BA doesn't happen.
Thanks again.