Stu
It would appear no name changes are allowed once travel is booked.
AIUI, if you were wishing to take someone then you'd have to cancel it and rebook assuming there is availability. There is also references to someone being in a HHA can't go with you if they are in a HA account but if they are on your Family and Friends list they can. (though there is no reference to it in the T & Cs but I'm sure I've read that before on here that has been allowed)
See 24. here
"Once travel using the Companion Voucher has been booked no name changes are allowed"
http://www.britishairways.com/travel...N_PHG_ALWAYSON