Rest assured that there's no need to wear a cravat in the Royal Enclosure at Ascot, Sandy; they're a rare sight and thought a bit eccentric; but you must never take your top hat off, unless seated in the restaurants, and you must wear a waistcoat. You must never lie down on the grass either, but that's another matter, less comprehensible; it's because "Her Majesty doesn't like it".
Certainly the wearing of a tie is not demanded as it once was and places that expected it have abandoned swimming against the tide. It depends more on image and expectation now. You would expect your solicitor, accountant, bank manager, or doctor to wear one because tieless suggests a casual attitude to matters of great importance to you.
Waistcoats? I have lots but they are only worn at race meetings, with a suit. They look and feel better than a pullover then and are almost as much part of the 'uniform' there as the trilby hat or the tweed cap (the cap for jumps, trilby for the flat is usual, if you bother with a cap)
Braces? Rarer now, people preferring belts. What has definitely gone is the suit made only for braces (Simon Cowell may have lots of those), with its high pointed back and high front to the trousers. A suit still looks better with braces though.