The first time I saw the scene, when I was absolutely innocent of any influence, when I had no idea really yet who either of them were or would be, and no idea about British QAF, I assumed, absolutely, from the way Justin said it, he was making it up to impress Daphne.
It's not bootstrapping, it's not disregrading canon, it was my completely spontaneous reaction to what I saw on the screen in front of me. I was literally shocked when I came to fandom and saw people who took Justin at his word.
The way he says it screams "I'm making this up out of a romantic fantasy" to me. It always did and it still does.
At that point, it had nothing to do with whether or not BRIAN would have said it, because we don't know Brian that well yet, but whether or not it sounded like JUSTIN was telling the truth. And it didn't, to me.
My acceptance of canon reinforces this, in fact, because QAF UK is not canon and Cowlip's intentions are not canon. The only thing I go by is what's in the episodes. It's clear when you take the whole five seasons into account, that Brian didn't say it. If he was in the habit of saying "I love you" just because some huge thing was going on and he was wasted... well, he'd have said it a lot more often, LOL.
What we saw and didn't see in Justin's face, body language, and tone of voice when he tells Daphne that is, of course, completely subjective. I'm just saying that I honestly saw him making up a pretty fairy story, based strictly on my observation of canon.