Funny you should ask this, I'm just about to finish in 2 months the BEng Civil Engineering at Bristol and am going to study a Masters in Hydrology at a different university.
From what you wrote the thing that sticks out to me is you asking if by scraping AAB you'd struggle at Bristol. I would say that would depend on your 1st year. Reason being, at Bristol, there will *absolutely/no doubt/100% be people who will work hard from day one while there will also be people who won't get off to a good start at all by not working and you have to choose which of those people you'd be. I know quite a few people who scraped into Bristol, did not work at the beginning (*the clincher), and that has dictated pretty much their whole university career. If you start working from the beginning, you would be absolutely fine with the course at Bristol and probably actually surprise yourself with how well you could do within your cohort. The worst thing is when people don't take the course seriously.
As to whether structural and transport are decent. I would say it's hardly worth thinking about, civil engineering is so broad and you'll begin to like things you didn't even know existed. If you're interested in civil engineering in general, that's enough to make a decision with.
I would definitely put Bristol down if you can afford to on your application in terms of still having 'back up' options. You might well get in with grades you thought might not get you in, don't take my word for it though!
Overall Bristol is an incredible experience - to be a bit more specific I mean it because it is insane how much I've learned personally in just 3 years. Last piece of advice - watch out for the toxic relationships!
All the best
D