These words are homophonous for speakers who ‘th-front’; that is, who pronounce the /θ/ sound in a word like thin with a more fronted sound, like /f/ in fin. This th-fronting is said to be characteristic of Cockney and Estuary English, as well as West Country dialects in the South West of England (Wells 1982: 96).
Our map shows no obvious dialectal patterning of this variable. There is certainly no prominence in the South; only 31% of London speakers pronounce these words the same, and it is even more scarce in the South West where only 23% of speakers do so.