Any local supermarkets? Tesco, asda etc.?
This is, we might say, "moderately exotic" for the British market so I would suggest a high-end supermarket such as Waitrose. Or a more specialist delicatessen. The average Tesco or Asda customer isn't likely to know what tapenade even IS, so I can't exactly imagine them stocking it.
Waitrose will definitely have it, or luxury food stores like those in the department stores Harrods and Selfridges or local delicatessens
Waitrose definitely sells it
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