Hoping someone can advise...

We are a party of 3 wishing to fly business. I have two upgrade vouchers which I wish to use to upgrade two of our party, and will pay full Avios for the 3rd person.

Online I cannot do this in one booking. Its important to me we are one booking as one of the party is a disabled minor. We've had horror shows due to split bookings in the past (odd person being moved etc.)

Will BA let me do it over the phone without charges, or is there a better way?