Patchy experiences, but Mandarin Oriental (MO) recognise BAEC (you can collect Avios) and a few times have made explicit reference to my Gold status on pre-stay communication / at check in etc. In the past Gold allowed for a room upgrade and enhanced service (received this twice at MO KL a few years ago) but more recently (5 stays at MO Shanghai and 2x MO HK in 2019) nothing explicit in terms of upgrade etc- probably as upgrade came anyway from booking direct via their frequent stay portal (aka their 'Fans of MO' programme: not a loyalty programme as such, but pretty generous once you've signed up and booked). Can't recall if any of this was formalised on the website (of either party), someone may remember better.