A kludgy way to work around it is to 'disable the internet'.
It may be simple if you only rely on wifi most of the time. Disable wifi from the control centre & then reply (iMessage uses the internet).
I wonder if a second way is to create two contacts, one for iMessage (which has the iCloud email etc) and one for SMS (with just the phone number) . I haven't tested this so messages may decide to use the phone number that is associated with the iCloud account. See if that works - it will cause you to have split conversations in the messages list.
Hopefully someone has a better idea. I also couldn't 'press & hold' to send as SMS on iOS 7, so that may just be "internet lore".
Apologies this may not help based on what you just posted about only using the phone number