BA tried to pull this on me a couple of months ago when I was checking in. I got out of it by pointing out that it was a first class ticket, issued as open on all segments except the first. So did they really expect me to pony up GBP25 each time I wanted to take a flight? I guess the check-in agent decided it wasn't worth arguing about and just took the coupon. I didn't get questioned on subsequent BA flights either. One alternative might be to have one of the other carriers in London write the segment stickers, though you will probably have to have the whole booking with them.