I don’t think the etiquette has ever been to sit down at big concerts, not since I’ve been going anyway which has been approximately 30 years. I can’t imagine that would be high energy for the artists either looking out at a seated crowd.
Have you tried speaking to security or the person in front to see if you can switch if you’re struggling to see althogether?
A lot of seated venues are quite steeply sloped to allow for shorter people to still see when other people are standing. Even the back row at the o2 a 5ft tall person should be able to see with the average height person standing in front.
I’m 5 ft 7 and wouldn’t expect someone 6 ft something to sit for my benefit at a concert. I’d look in the gap between the two people in front.
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I’ve not been to Birmingham but managed to switch from standing to seated at the o2 due to back problems. It’s always worth getting there early and asking customer services.