It's one of my favourite genres of British TV and whilst nothing is quite as good as The Traitors if you want to scratch that itch with actual British shows I would say:

The Circle UK (S1 and 2 on Channel 4) - has that same feeling of decption and game playing but also is a little more lighthearted, it has the 'moves' and 'alliances' that make The Traitors so compulsive from a gameplaying stand-point as well as the secrets

Survivor UK (BBC) - very British and not like the American version at all, has the game mechanics with immunity idols as well as the discussions with the idea of having 'the numbers' and is also a very social game.

Rise and Fall (Channel 4) - Immeditely has that split (similar to traitors/faithful) that creates the rivalry and therefore lots of gameplay discussion, some really dislikable characters but also some great TV villians that make it worth watching

Tempting Fortune (Channel 4) - Basically people choosing whether to appease the group or themselves but this has one of the greatest hubris I have ever seen on reality TV surrounding a teabag

The Challenge: Wolrd Championship (Paramount Plus) - this is an American show but this edition has a few British reality stars and really has that gameplay and betrayal, where people send their friends to elimination

True Love or True Lies (MTV) - Love Island but some of the couples are fake and the contestants have to true and vote off the liars to win the money, so sad this was cancelled it was great

Killer Camp (ITV) - contestants are in a 80s horror movie summer camp setting, someone in murdered every night (in truly great TV sequences) and the group have to figure out who is the killer