I'm assuming the dress code was mentioned when they made a courtesy call in midweek to apologise for their chairman not coming to represent them?
It's not hard to work out that there may be other notable guests, even potential investors there who the club need to make a good impression on. A high standard of dress code is a way of ensuring that. It's just common courtesy.
As for what would they do if Dale Vince turned up like that? Well, exactly the same as we're not as sycophantic as you. And just to let you know, when Dale Vince meets with people, and at his level you're talking multi million pound financiers/bankers he wears a shirt and tie, don't worry about that.
Running for MP I notice, if successful you know full well typhus be suited and booted every day.
Utterly disrespectful.