What is the best time to arrive at an All Time Low concert in Brixton on the 13th March?

Well, if it's general admission and you can stand wherever you can I'd get there as early as possible. It's too bad you are at the mercy of your dad but just be thankful he is taking you and willing to go through that for you! You can always try to work yourself towards the front once All Time Low actually goes on. As for the merch it's best to get after the show so you don't have to hold it the entire time (unless you are taking a purse/bag things will fit in) but check with the merch table to make sure they will be there after the show as some shut down early. I've fallen victim to this. Most importantly have a great time! :)