Unfortunately, in the current 'tournament scene', there are no 'forfeit' rules written in. I feel there should be, but that's for a design thread, as they would be subject to abuse.
I would REALLY like to be able to concede a game. At this past Adepticon, my dice luck was so bad, I thought I was being punished for sins of a past life. But I kept playing, to help my team and because I entered into a social contract. Unlike a casual environment, your opponent in a tournament is under no burden to make sure you have a good time. They must be fair, they must be polite, but they have the right, perhaps even the moral authority, to do their best to
crush you like bug.
I would love to, as in Chess or Go, concede a game. But I can't, because that will unfairly give my opponent full points she or he did not earn, and that is unfair to the other people in the tournament. Chess has a simple win-loss-draw outcome.
40K tournaments can have entire ranges of battle scores. You may not be able to affect their army, but maybe you can deny them full points by going to ground on objectives, or denying them other scenario points.
Yes, it isn't fun. Maybe you can shift your mental focus to start grunting like Rambo as your last guy suicide charges (note: Catachans do this best), to keep your opponent more than 3" away from an objective. It isn't easy to make this shift, especially if you've been rolling no higher than a '2' all day. I, myself, could not make this mental shift. But I know I should have (hindsight being 20|20).
It is a tournament, and it is entirely possible that you have paid for a day (or a weekend) of getting your

kicked. These are the risks you take.
Be prepared, mentally. Have a plan of action for how you will respond to being crushed. Maybe the top-tier players don't have to deal with this, but us mortal plebes do.
And, after you've completed the game to the best of your ability and denied your opponent the three points they needed to take 'best overall'? THAT is your moral victory. That is your takeaway. You were the spoiler. You were their Arnhem Bridge.
Go tell the Spartans.