Probably not. I try and keep a balance of smart/casual when seeing West End shows. If I'm wearing jeans, I make sure they're clean and not torn or anything and I'll always wear a nice top with them rather than a t-shirt. I try not to wear trainers as far as possible but I kinda forgot to take any nice shoes last time I went to London. XD But really, it's like Broadway in that people will wear anything, except I've not heard any horror stories of sweatsuit-wearing matriarchs carrying buckets of chicken and fifteen rowdy offspring.

Hell, there were some guys wearing jeans and t-shirts in the massively expensive seats at 'Follies In Concert' last month. So it's personal taste. :3