Just a bump from my original post, Next year the plan is to now Fly from a London Airport to Orlando, get an internal flight from Orlando to Vancouver (for a cruise) and then fly back from Vancouver to London.

Im guessing that as my Companion Voucher was earned on my Premium Plus BA Amex that it doesn't change anything and I just do the same procedure of booking my outbound flight and then calling the US number (ive setup and got cheap calls on Skype, so no large fees this time) to add the flight from Vancouver to London.

Is it likely to make much difference to the required Avios amounts?