The realisation of /θ/ as /f/ in English (and similarly for its voiced equivalent) has long been thought of as a Cockney trait, made fun of by generations of comedians, and bemoaned by countless traditionalists as heralding the demise of ‘proper’ English. Less well-known is its occurrence in other varieties of English, such as certain Scots dialects, but it has never been recognized as an alternative acceptable pronunciation in standardized British English.
How long can we sustain this? It’s now heard from all sorts of otherwise apparently well-educated people. Is it to be classed as a speech defect? Lucy Worsley, the recently popular presenter of BBC television programmes on interior design (“If Walls Could Talk”), and Chief Curator at Historic Royal Palaces, uses /f/ and /v/ regularly, and yet she is clearly a very well educated woman.
How long before the various pronunciation dictionaries have to start including /friː/ alongside /θriː/, and /ˈfɜːvə/ together with /ˈfɜːðə/. This will create a whole new set of acceptable homophones, including fervour ~ further, free ~ three, and sliver ~ slither, the last of which are already confused by hyper-correcting Londoners who may talk of a ‘slither’ of wood.