Jack Britten is a non-canonical character created for ITV's 2007 adaptation of the novel At Bertram's Hotel. He and Joel, his twin brother checked into Bertram's Hotel at around the same time as Miss Marple near the beginning of the show.
Jack and Joel looked very much alike and both were in fact portrayed by the same actor, Nicholas Burns. The brothers used their strong resemblance to each other to further their purpose which was to steal jewels from guests of the hotel. In one such operation, Joel appeared at the dining room asking for his brother. He then changed clothes and reappeared as "Jack". This gave Jack an alibi for almost an hour while he stole into the room of Selina Hazy, broke into the safe and stole her jewel necklace. However Miss Marple saw through this when she noticed that Jack was left-handed and always wore his wristwatch on the left hand while Joel was right-handed and wore his wristwatch on the right hand. The two Brittens seen in the dining room wore their wristwatches on the right hand. Jane Cooper, helping Miss Marple, also found clip-on ties in their bedroom, ideal for quick clothes changes which was the basis for the deception. Their modus operandi was exposed by Miss Marple and Jane Cooper during the denouement. Joel returned the necklace to Selina Hazy and both brothers were arrested and taken away.
Their heist at Bertram's was likely not their first. In an early scene, one of the brothers was seen reading a newspaper whose headline reported a spate of jewel thefts in London hotels.