In mid-September, on the Goodwood Estate in West Sussex (near Chichester) there is an annual gathering of vintage motor vehicles and fans of the fashions of the 1940s, 1950s and 1960s. This is known as the Goodwood Revival, and it is one of the few places where those who are 'dressed up' outnumber those who haven't made the effort. As with many annual events under current COVID circumstances, plans for the event may be subject to change.
This is the website https://www.goodwood.com/flagship-events/goodwood-revival/
Note also that the organisers are keen to promote the wearing of quality period outfits with Best Dressed awards each day (Further information can be found on the Revival website).