Well, Simon is always underdressed, but I think his “fashionable” 5 day stubble, seeing as it is grey now, really doesn’t do anything for him. He looks more homeless than cool.
Thus is different. Usually he wears a nice t shirt and looks clean, on BGT, he does look like a homeless person. I like the interaction between him and Walliams and Walliams’ ‘act’. Once he figured out that it annoys Simon he really camps it up around him! :)