Ceremony of the Keys
It is not the longest ceremony around, but definitely one of the more meaningful, and certainly the one that has been going on the longest. The Ceremony of the Keys takes place every evening at the Tower of London and was, up until quite recently, free of charge. In order to defray the cost of the booking system however, it now costs £5 per person, which is still a great deal. The tickets are difficult but not impossible to obtain especially if you keep an eye on the website: https://www.hrp.org.uk/tower-of-london/whats-on/ceremony-of-the-keys/#gs.pzi8pi Check frequently to see when the next tranche of tickets is released (usually at the beginning of a month for the following month) and you will be able to snap them up.
So what exactly is this Ceremony of the Keys? It is the official locking-up of the Tower of London for the night, and it happens privately inside the Tower at 10pm. Only a small group from the public is admitted to watch every evening and you could be in this group, to witness the Ceremony that has been going on for, oh, the last 700 years or so. There is a sign next to the gate where you meet and first listen to the Yeoman Warder's background explanation and description of the events that will happen before proceeding into the Tower. No photos are allowed, unfortunately, so just savour the experience of an armed escort processing with the keys up to the gate and then being challenged by a sentry upon their return. We were lucky to witness a particularly vocal challenge by the sentry on the night we were there recently, and there were also a large number of VIP guests in attendance as well. And even if you have been there before, it's a different ceremony now as the keys in questions are now the King's Keys instead of the Queen's Keys!